Major depression was meticulously described by Hippocrates nearly 2500 years ago. Melancholia, a subtype of depression, is the only word which was coined by Hippocrates based on his theory of the four humors (melancholia: black bile), and that is still used today to describe a disorder that is part of contemporary official diagnostic systems. This book covers major depressive disorder, specifically unipolar depression, a condition that we conceptualize as a common and complex disorder of gene– environment interactions for which the specific genetic and environmental substrates are still unknown. These words are simple but are loaded with meaning, as follows. Common – depression is highly prevalent worldwide. As the disorder is episodic, lifetime rates are higher than point prevalence. Even though figures vary worldwide, depression is indeed very common.