Circadian physiology provides a useful context in which to study the neural control of behavior. When applied to Drosophila, the studies take further advantage of the powerful genetics and cellular resolution available in this model system. Rhythm-generating neurons can be identified and manipulated, and the neural circuits analyzed in-depth. We are interested to learn how the brain uses circadian timing information to organize daily rhythmic locomotion. This talk will review our recent in vivo imaging measurements of the pacemaker circuit throughout its daily cycle of activity. We propose a model whereby the intrinsic clocks of pacemaker neurons and their cellular interactions together create proper timing cues (outputs) by which rhythmic behaviors such as sleep & locomotion may be properly aligned across the solar day. Finally, we will discuss on-going studies of the neuromodulatory peptidergic cell-signaling that is critical to the functions of this rhythm-generating circuit.