Twin research has a long history and plays a central role in the ongoing exploration of the influence of genes and the environment on human development. Recent advances in molecular biology and the ability to identify the role of specific genes in human development raise new questions about how genes and the environment influence behavior. At the same time, societal changes have led to an explosion in multiple births that can largely be attributed to advances in reproductive technology. This article describes the assumptions underlying twin research methodology, explores the current context for twin research, and summarizes the major themes that have emerged from large-scale twin studies conducted over the past several decades.