Understanding how individuals differ from one another and from closely related species is a fundamental problem in biology. Recent evidence suggests that much of the variation both within and between species is due to differential gene regulation. Here we review differential gene regulation focusing on evolutionary-developmental (evo-devo) biology, global comparison of genomic sequences, whole-genome gene expression, and transcription factor (TF) binding profiles. We also explore the relationship between divergence rate of regulatory sequences, coding sequences, and TF binding events using several different measures and discuss their implications in the context of evolution of regulatory networks. Finally, we discuss the current status and future challenges in relating regulatory variation to the divergence across and within species.