Perception plays an essential role in the control of action by coupling behavior to the environment. The visual control of posture and locomotion provides one of the best-studied cases in point. By presenting optic flow patterns to a walking subject, we can identify control laws relating optical information to the parameters of action, which act to stabilize the relation between the agent and its environment. We have found that postural adjustments during walking are specific to the direction, amplitude, and class of visual stimulation, and reveal a reliance on motion parallax components of optic flow. We are currently using virtual reality techniques to study the visual control of walking, including steering toward goals, down corridors, and avoiding obstacles. Optic flow tends to dominate when available, but other variables also contribute to the control of locomotion. We are investigating the conditions under which these different control strategies, and transitions between them, are manifested in behavior.