This document summarizes key concepts in psychiatry related to mood disorders such as depression and bipolar disorder. It discusses mood and affect, defines major depression and manic/hypomanic episodes, and explores the epidemiology and potential biological and psychosocial factors involved in these conditions. Specific brain regions implicated include the prefrontal cortex, anterior cingulate cortex, hippocampus, and amygdala. Genetic and environmental influences are also reviewed.