Beginning with his doctoral dissertation in 1950 which introduced the study of event perception and the application of vector analysis to perception, Gunnar Johansson has been a seminal figure in the field of perception. His work on biomechanical motion in the 1970s challenged conventional notions and stimulated great interest among experimental psychologists and students of machine vision. In 1989 Johansson published his latest theoretical synthesis, the optic sphere theory, an innovative conceptualization that goes beyond his earlier proposals. This volume presents -- for the first time -- an extensive precis of the out-of-print classic 1950 monograph prepared by Johansson. It also includes a representative set of Johansson's important publications produced over the ensuing four decades. These papers served as the springboard for a set of original essays by a distinguished group of North American and European scientists. Part critical commentary, part elaboration, and part seeking new directions, the entire collection makes for a singularly rich treatment of the perception of objects and events.