Obesity is one of the most common diseases today, which increases the risk of many other physical diseases, but also mental disorders. Moreover, people with mental disorders are more likely to be overweight or obese than the general population, e.g., the prevalence of obesity among the general population is 20-30 %, and among people with psychiatric disorders is up to twice as high. Life expectancy in people with some psychiatric disorders is shorter than in the general population, which is also attributed to the impact of obesity on the increase in cardiovascular disease mortality in these patients. Considering the great bidirectional connection between these diseases, it is important to notice what are the mechanisms of this connection in order to better intervene, i.e., prevent their occurrence or treat them if they are already present. Apart from the factors related to the lifestyle of people with psychiatric illnesses (unhealthy eating habits and sedentary lifestyle), weight gain arises due to various biological factors and as a side effect of some psychopharmacs. People who are overweight and obese are more likely to have more severe forms of diseases and worse outcomes of treatment for psychiatric disorders, and are more likely to stop their therapy compared to people with normal body weight. Various behavioural and pharmacological methods of treatment are used to control and reduce body weight in psychiatric patients.