Genetic epidemiology explores the interrelationship of genetic and environmental risk factors in which genes are measured indirectly in ways that reflect aggregate effects "averaged" across the entire genome. This chapter describes the principles and methodology of psychiatric genetics using four-paradigm framework: basic genetic epidemiology, advanced genetic epidemiology, gene finding, and molecular genetics. Each of these paradigms has strengths and limitations, and they are in a process of dynamic interaction with each other. Genetic epidemiology has proved a reliable method to answer basic questions about the overall importance of genetic risk factors for psychiatric illness. The advanced genetic epidemiology paradigm has been used to study the relationships between neuroticism and depression. Molecular genetics is an entirely laboratory-based discipline applying a range of modern methods from genomics to neuroscience to try to identify and then trace pathophysiological pathways.