Immersive virtual reality is the research tool those of us who study human perception and action have been waiting for, because it enables us to perform rigorous experimental studies of natural behaviour. First off, it allows us to easily manipulate realistic visual environments while collecting continuous measures of ongoing behaviour. But its greater potential lies in the ability to test psychological theories by manipulating the world in impossible ways. I will describe several studies in which we break the laws of physics and optics to investigate some interesting visual-motor control problems: (a) How do baseball players catch a fly ball? (Manipulate gravity) (b) How do people guide locomotion? (Manipulate the optics) (c) What information is used for path integration? (Manipulate the visual-motor gain) (d) How can you tell if you're being stalked? And is a zig-zag path the best escape? (Manipulate the behavior of virtual agents) William Warren is Chancellor's Professor of Cognitive, Linguistic, and Psychological Sciences at Brown University. He earned an undergraduate degree at Hampshire College (1976) and a Ph.D. in Experimental Psychology from the University of Connecticut (1982). He uses virtual reality techniques to study the visual control of human locomotion and navigation, with funding from NIH and NSF. Warren is author of over 85 research articles and chapters, the editor of two volumes, and the recipient of a Fulbright Research Fellowship, an NIH Research Career Development Award, and Brown's Elizabeth Leduc Teaching Award for Excellence in the Life Sciences.