How does a nervous system control an animal’s behavior?  We are investigating this question by manipulating and monitoring neural activity of large populations of neurons with cellular resolution in the brain of a simple transparent organism and correlating the observed neural dynamics with behavior. I will present a suite of optical tools to control and record activity in the nematode C. elegans as it moves, including the first instrument to perform whole-brain calcium imaging with cellular resolution in an awake and unrestrained behaving animal. We have used these techniques to gain insight into the underlying neural mechanisms behind mechanosensation, forward locomotion, and the C. elegans escape response. These measurements are a critical first step for investigating neural coding of behavior, decision-making and transitions between internal brain states.