This chapter is organized in the following manner. First, a detailed consideration of the ocularmotor system is provided. This section must be reasonably detailed in order to justify the measures recommended. Second, a series of clinical tests are detailed that are useful in indicating whether the ocular-motor system has been placed under stress. These tests are used generally to explore ocular-motor status before and after using a particular virtual environment (VE) system for a given period of time. Finally, a different approach to investigating the impact of VE systems on the ocular-motor system is discussed. This final section describes experimental results that highlight some interesting features of the ocular-motor system. These results are used to illustrate the fact that VE systems can help us understand the nervous system and to argue that the most profitable aftereffect measurements result from paradigms that probe the manner in which the nervous system adapts to virtual environments.