This chapter introduces factors contributing to depressive disorders. It begins by clarifying terms that may have been ambiguously defined. Next, stratified medicine, precision medicine, and personalized medicine are discussed. The various databases that bring the power of modern investigative techniques to bear upon precision medicine are identified. The National Institute of Mental Health Research Domain Criteria framework for new approaches to investigating mental disorders is detailed. Then, PsychENCODE Consortium results, other databases, and new investigatory techniques are discussed to expand understanding of depressive disorders. Through this exploration, this chapter aims to provide an overview of the multifaceted nature of depressive disorders and the evolving landscape of psychiatric research.