Of the three defining properties of circadian rhythmicity--persisting free-running rhythm, temperature compensation, and entrainment--the last is often poorly understood by many chronobiologists. This paper gives an overview of entrainment. Where have we been? Where are we now? Whence should we be going? Particular emphasis is given to a discussion of the Discrete vs. Continuous models for entrainment. We provide an integrated mechanism for entrainment from a limit-cycle perspective.