At first sight, it is straightforward to answer what is a disease. Most of us feel we have an intuitive comprehend the idea, reaching mentally to memories of cancer, diabetes and stroke or heart diseases. However, when we look at through any medical dictionary, a satisfactory definition of disease is surprisingly difficult. Moreover, the definition of disease as the opposite of health is not much help and even giving the definition of health is also equally tricky. The World Health Organization defines health as a state of complete physical, mental and social well-being, not only the absence of disease or infirmity. Medical dictionary defines a disease as an illness which brings about an interruption cessation, or disorder of bodily functions, system or organs. A disease is indicated by at least two of following criteria: (1) a recognized etiologic agent; (2)an identifiable group of signs and symptoms;(3)steady anatomical changes of known body structure. With this in mind, whether addiction meets this definition must be evaluated in order to call addiction as a disease (Gorski, 2001). Addiction is defined as a condition which results when an individual ingests a substance (alcohol, cocaine and methamphetamine) or engages in an activity like gambling, sex and shopping that can be pleasurable, however, the continued use or act of which become compulsive and interferes with daily life responsibilities such as work, relationship and health. To diagnose addiction , it must meet at least three of these criteria: tolerance, withdrawal, limited control, negative consequences, neglected activities, significant time or energy spent ( Thombs, 2006). It is unknown how or why some individuals become addicted to drugs and others do not. Drug addiction is a complex illness. Quitting is very difficult because compulsive drug abuse changes the brain patterns. Also, heredity plays a role in addictive behavior. However, some people think that addiction cannot account as a disease due to the fact that it is caused by the persons’ choice to use drugs or alcohol. Consequently, whether addiction is a disease or not is a controversial issue. As a result, in this paper, addiction as a disease will be discussed in the light of recent articles. Addiction is known as a chronic disease of brain reward, motivation, memory and related circuitry. It is also characterized by impairment in behavioral control, unable to consistently abstain, craving, decrease recognition of important problems with one’s actions and giving a much higher priority than other behaviors, a dysfunctional emotional response as well.    Addiction impacts neurotransmission and interactions through reward structures of the brain which include the nucleus accumbens, anterior cingulated cortex, basal forebrain and amygdale. Therefore, dysfunction in these circuits poses biological, psychological, social and spiritual problems. Addiction changes brain structure especially drug addiction. To illustrate, long-term drinking causes structural changes in the human brain, which leads to shrink human brain size (Leshner, 2000). Autopsy results clearly demonstrate that chronic drug users have lighter and smaller brains than other individuals of the same age and gender. In addition, this shrinking impact in living users can be obviously seen through non-invasive medical tests such as magnetic resonance imaging (MRI), positron emission tomography (PET) scans, and computed tomography (CT) scans. Moreover, the same techniques show how addiction damages brain cells. For instance, methamphetamine harms cells which generate dopamine, which is a chemical in the brain that is responsible for creating feelings of euphoria. The use of methamphetamine may trigger aptosis, in which cells in brain self-destruct.             Moreover, addiction can significantly damage brain functions. According to the National Institute on Alcohol Abuse and Alcoholism, alcohol addiction causes some kind of cognitive impairment. The study results demonstrate that alcohol dependent inpatients show significantly low fluid intelligence, visualspatial memory, working memory, set-shifting flexibility and planning or organization (Manning et al, 2016). Furthermore, studies of chronic alcohol exposure animal models also determine neurological deficits of alcohol addiction. Chronic alcohol exposure decreases proliferation of neural stem cells and survival rate of newborn neurons. Besides, alcohol-fed mice show significantly decrease in the dendritic spine density of newborn neurons (Golub et al, 2015). One of the primary neurotransmitters playing an important role in addiction is dopamine. All the major drugs such as nicotine, cocaine, and heroin augment dopamine levels. Even though the excess dopamine produces powerful feelings of pleasure, the excess levels have a significant role in developing addiction. This is because the excess level harms the biological concept of homeostasis. When the brain is consistently exposed to artificially high levels of dopamine from drug abuse, it becomes to depend on a drug in order to maintain homeostasis and function normally. This creates the problem. Drug addicts experience symptoms such as depression, withdrawal and fatigue when the extra dopamine supplied by drugs lacks. Therefore, the addicts see the merely way of relief from these feelings is to use more and more drugs. This plays an important role in developing tolerance and craving. Also, chronic exposure to drugs changes the prefrontal cortex, which manages motivation, inhibitory control and choice (Thombs, 2006). In the light of this information, addiction is considered as a brain disease because it changes the brain’s structure and how it works. Also these changes pose mental and physical problems. Even though there is a lot of neurological evidence support addiction as a disease, some of people claim that addiction is not a disease because brain changes in addiction are not abnormal. The changes in the brain evidenced by the brain scans of addicts do not represent malfunction brain according to research into neuroplasticity. When people practice doing and thinking anything excessively, there are changes in various areas of the brain such as growing or strengthen of different regions and neuronal pathways, and developing new connections. Therefore, there is not abnormal change in addicts’ brain. Individuals learn addiction within neuroplasticity, which plays a role how person learn everything. People who are addict lose some of that plasticity for a while so they maintain their addiction (Lewis, 2015). The science of neuroplasticity has mostly demonstrated that physical experience and inputs from outside world can cause the brain change. In other words, the circuits expands or strengthens when they are used frequently and also those can be shrink or weakening when rarely engaged (Begley, 2002). To illustrate, Begley and Jeffrey (2002) proposed that a specific area of the brain’s hippocampus is responsible for creating directional memories and a mental map of the environment. Their research was conducted to compare the brains of London taxi drivers and non-taxi driver. The results demonstrate that there was a very significant difference between drivers and non- drivers. They found that a taxi driver who works many years has smaller front of hippocampus and larger posterior. This shows that there is a positive correlation between time spending as a driver and right posterior hippocampus. In addition, learning to play the piano well also changes the brain structure. There is a significant difference between a good piano player and non-piano player. Therefore, they claim that addiction is a choice instead of brain disease. However, the brain changes in addicts cause negative effect on their psychological and physical health, social life as well. For example, as mentioned above, addiction interrupts the brain function and leads to cognitive impairment. Also, addict people can experience depression, fatigue and withdrawal symptoms when they stop to involve in activity or use drugs. Yet, Begley and Jeffrey explanation and results do not cause any negative effect on people mental or physical health and even help to improve the mental ability. That is why addiction is a brain disease which is progressive and can cause disability or premature death. The other biological underpinning of addiction as a brain disease is genetic predisposition. Not all individuals who use drugs or engage in an activity will experience the changes in brain function or structure described above. Some persons can involve in substance use occasionally and stay occasional users whereas other individuals begin using substance drugs casually and start to progress inevitably to addiction. The reason behind this is unknown but researches demonstrate that heredity plays an important role in development of addiction (Enoch and Goldman, 2001). There are a lot of studies which looked at genetic underpinning of addition in identical twin pairs and fraternal twin pairs. To illustrate, when one identical twin is alcohol addict, the other twin can face more likely to be addicted. However, if one non-identical twin is addicted to alcohol, the other twin would not necessarily be alcoholic (Prescott and Kendle, 1999). As a result of this, the study determines 50-60% of addiction is a possible existence of the genetic predisposition based on the differentiation between the identical and fraternal twins. Moreover, a study determined that children of addicts have higher tendency to develop an addiction. The study was conducted by comparing 231 participants who had alcohol or drug addiction with 61 participants who were not drugs or alcohol addict. Then, the first-degree relatives of participants such as parents, siblings and children were determined. The results of study demonstrates that if a parent is a alcohol or substance addict, the child is an 8 times more likely to develop an addiction (Merikangas et al, 1998). Here, there is an important question should comes to people’s mind. Is there a specific gene that increases susceptibility? There is a collaborative project known as the Collaborative Study on the Genetics of Alcoholism (COGA) which is mapping DNA sequence. COAG conducted many researches to identify the specific genes that underlie alcohol addiction. COAG findings demonstrate that alcohol susceptibility is more likely to be linked several genes which is known as polygenic. In other words, alcohol addiction is not the result of a mutation in a single gene (Thombs, 2006). It is clearly seen that addiction is not an inherited disease which is caused by single gene. Instead, addiction is a complex illness caused by a bunch of genetic and environmental variables. Also, genetic risk factors can augment or decrease risk for addictive behavior. On the other hand, Lewis (2015) argues that because addiction is starting with a choice, it is a purely learning phenomenon or behavioral problem that needs to have great willpower and high motivation to change. However, addiction is caused by the reasons which are similar to most major diseases like changing brain pattern and specific genes. When we consider heart disease, it is partly caused by genes and partly by poor life style choice like bad diet, smoking and lack of exercise.  Other common disease such as adult-onset diabetes and some forms of cancer have the same reasons to develop. Besides, anorexia and obesity also result from life style choice. Thus, do we accept these diseases as a learning behavior problem? Of course we do not. In conclusion, addiction is known as a compulsion to use a certain substance or engage in specific behavior in order to feel good and sometimes use more than you would like to use and also maintain to use or participate despite of negative consequences. Individuals use drugs or alcohol and engage in activities to escape, relax or to reward themselves. Over time, they begin not to enjoy life without using drugs or engaged in activities and they cannot control their behaviors. Addiction especially drugs changes how the brain works, and also these brain changes can cause psychological, physical and social problems for a long time. Besides, heredity plays a significant role in development of addiction. That is why addiction can meet disease definition. With these in mind, addiction should be a brain disease. Despite of these evidences, some scientists claim that addiction is a learning phenomenon due to starting with voluntary. Also, the science of neuroplasticity demonstrates that the brain can change during learning. In other words, the circuits in the brain get strengthened when used occasionally or shrink if they rarely activate. To illustrate, there is a significant difference between people who play piano very well and those who do not. However, the brain changes in people with addiction cause psychological and physiological problems such as depression, fatigue and cognitive impairment as well. Moreover, genetic predisposition is another reason for addiction to be accepted as a disease. Like some of major disease, addiction has genetic risk factor and life style choice. To illustrate, some types of cancer such as lung and skin cancer and type 2 diabetes result from genes and life style choice.     References Begley, S., & Schwartz, J., ( 2002). The mind and the brain. Harper Collins Enoch, M. A., & Goldman, D., (2001). The genetics of alcoholism and alcohol abuse. Curr Psychiatry Rep. p. 144-51 Golub, H. M., Zhou, Q., Zucker, H., McMullen, M. R., Kokiko-Cochran, O. N., Ro, E. J., & ... Suh, H. (2015). Chronic alcohol exposure is associated with decreased neurogenesis, aberrant integration of newborn neurons, and cognitive dysfunction in female mice. Alcoholism: Clinical And Experimental Research, 39(10), 1967-1977. doi:10.1111/acer.12843 Lewis, M., (2015). Recovery ( like addiction) relies on neuroplasticity. Retrieved from: https://www.psychologytoday.com/blog/addicted-brains/201512/recovery-addiction- relies-neuroplasticity Manning, V., Teo, H. C., Guo, S., Wong, K. E., & Li, T. (2016). Neurocognitive functioning and treatment outcome following detoxification among Asian alcohol-dependent inpatients. Substance Use & Misuse, 51(2), 193-205. doi:10.3109/10826084.2015.1092985  Prescott, C. A., & Kendler, K. S., (1999). Genetic and environmental contributions to alcohol abuse and dependence in a population-based sample of male twins. Am J Psychiatry. 156: p. 34-40.  Merikangas, K. R., Stolar, M., Stevens, D. E., Goulet, J., (1998). Familial transmission of substance use disorders. Arch Gen Psychiatry. 55: p. 973-9. Thombs, D. L., (2006). Introduction to Addictive Behaviors. The Guilford Press. New York   Gorski, T. T., (2001). Disease model of addiction. Retrieved from: www.cityvision.edu/courses/coursefiles/306/gorski.pdf