This paper describes a software environment for visualizing and segmenting volumetric data sets such as CT, MRI and the Visible Human data set. The goal is to produce an intuitive environment where the expert knowledge of the end user can be employed to directly guide visualization and segmentation of the data. The environment is built around the Fakespace Immersive Workbench (TM), which provides the user with the illusion that the data set volume resides in the space directly above the workbench surface. Using a position/orientation-tracked probe the user is able to interact with the visualization algorithm and segment the data set to expose features of interest. Segmentation can be performed in either the ray space of the volume rendering algorithm or the coordinate space of the data volume itself. The segmentation results can be saved and used for other purposes including the construction of polygonal models.