Usability evaluation of virtual environments (VEs) to date has received very little attention in VE literature. While awareness of the need for usability evaluation of VEs appears to be on the rise, the techniques needed to perform eecient, meaningful usability evaluation of these environments are not yet available. In particular , there have not been any widely developed, much less accepted, methodological approaches to assessing VE usability. As such, researchers interested in VE usability are left to performing ad-hoc assessments or in-house evaluations with little or no scientiic basis for their approach. A focus of our work is creating, studying, and optimizing human-computer interaction in VEs. More speciically, we are interested in developing methods to guide systematic usability engineering of these environments. This paper presents a novel method to guide the usability evaluation of virtual environments, and describes its application in assessing an immersive medical visualization VE, called Crumbs. This paper also discusses the use of eye tracking as a mechanism for monitoring human activity in VEs, and more speciically, as an instrument to facilitate formative usability evaluation of fully immersive VE interfaces. Eye tracking, to our knowledge, has not yet been incorporated into fully immersive VEs, much less systematically studied in this environment. The eye tracking systems add a new feature and data source to VEs, and greatly enhance the study of cognitive, physical, behavioral, and psychophysical properties in VEs.