This document provides an introduction to a special issue journal on how gonadal hormones affect behavioral and neural development. It summarizes several studies showing that hormones like testosterone and estrogen influence behaviors in humans and other species. For example, females exposed to high testosterone levels prenatally tend to show more male-typical behaviors. The introduction discusses themes across the special issue articles, such as how hormones have both organizational effects during development and activational effects in adulthood. It also provides background on the range of methods used to study hormone-behavior relationships.