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There is no doubt that the Internet has become the most useful tool for the young and adult people. It has an opportunity for people to explore knowledge and to be in touch within the contacts all over the world, as its usage is quite easy and is fast. However, the overuse of Internet, namely spending most of time on the Internet, has some cons on daily activities in respect to the interfamily communication and emotional stability. Nowadays, the Internet addiction is a widely spreading problem. This disorder has been reported both among Western and Eastern societies containing adults and adolescents. Researchers report that Internet addiction is related to a variety of psychiatric disorders. The results several studies show that the adolescents, who are addicted to the Internet, have more deficiency in attention rather than normals, and have symptoms of attention deficiency and hyperactivity disorder (ADHD), depression, social phobia, and hostility. The comorbidity between two disorders shows a higher relationship by chance than expected. Hence, it indicates an existing mechanism or mechanisms to provide affiliation. It is prominent to perceive such mechanisms to have an effective treatment for both disorders. In order to explain the affiliation between addiction and a psychiatric disorder, four possible mechanisms have been specified; firstly, the psychiatric disorder results in the symptoms of the other psychiatric disorder. Following, firstly, the psychiatric disorder results in, contributes to, or deteriorates the symptoms or course of the addictive disorder. Secondly, the addictive disorder leads to, contributes to, or further deteriorates the symptoms or course of other psychiatric disorders. Third, there are fundamentally shared biological, psychological, or sociological mechanisms by addictive and psychiatric disorders. Other components associated to sampling, evaluation, investigation, study conceive, or analysis outcome in inappropriate over-estimation of the comorbidity. The most well-known comorbid conditions of Internet addiction are mood disorders. Internet addiction is shown to be significantly associated with depression and suicidal ideation. In adolescents, Internet addiction may lead to depression and/or suicidal ideation. In the other hand, adolescents with depression or suicidal ideation may develop Internet addiction to cope with stressful life events. It is, therefore, important to understand such mechanisms; there may be a complex transactional relationship between Internet addiction and depressive symptoms. An association between Internet addiction and social anxiety has also been found among Asian adolescents. Internet use may help to adolescents with social phobia in avoiding to face-to-face interplay with peers. Researchers suggest that hostility is associated with Internet addiction in adolescents. Moreover, adolescents with Internet addiction are more likely to manifest aggressive behaviors. Studies demonstrated that adolescents with Internet addiction had higher ADHD symptoms. Treatment of ADHD symptoms could provide a decrease in the deteriorating effect of co-morbid symptoms with Internet addiction. Moreover, depression, ADHD symptoms and social anxiety should be carefully appraised and treated in adolescents with Internet addiction. Although the effects of the treatment strategies of comorbidities was beyond the scope of this presentation, it appears reasonable to suggest that effective treatment for comorbidities are required for patients with internet addiction.