Depression is regarded as one of the most abundant diseases worldwide with deleterious, even life-threatening consequences for patients. Symptoms and origins of this mental illness are very diverse and not always clearly recognizable. There exist various hypotheses of underlying mechanism(s) that trigger the development of depressions. Due to the interaction of several factors, depression is regarded as a multifactorial disease. The first part of my diploma thesis comprises a brief historical overview and selected basic aspects of the disease such as classification models, underlying mechanisms, symptoms, pharmacotherapy, and alternative treatment. The major aim of my thesis was to investigate genetic factors that contribute to the development of depressions. The search for predisposing genes using linkage and association studies has made considerable progress in recent years. My research suggests that in addition to environmental factors, variability of distinct genes involved in neuronal transmission plays an important role in the development of these diseases. Family and twin studies suggesting heritability of depressions. There are many open questions regarding the genetic component of this disease. But it is clear that there is no isolated “depression gene“.