It is now axiomatic that challenging behaviors are defined more profitably by their functions (their motivations) than by their topographies (what they look like). The notion that challenging behaviors can be defined on the basis of their function has led in the past 30 years to a dramatically reconfigured approach to assessment and intervention. This article provides a description of the conceptual and historical foundations of function-based interventions, with an emphasis on the significance of these developments for the field of early childhood intervention. In addition, the authors consider some of the major issues related to challenging behaviors for current practice and future research.