This article presents the state of the art and practice in system and software variability. It describes the trends driving the increasing importance of variability management, proposes a research agenda, and provides an overview of the articles in this special issue. The Web extra at http://youtu.be/80DAUGahNAY is an audio recording of Davide Falessi speaking with Guest Editors Rafael Capilla and Rich Hilliard about the state of the art and practice in system and software variability.