Register is an interactive graphics application. It can display one or two volumes (typically MR or PET), and the merged image of the two. There are three viewports for each volume, showing tranverse, sagittal, coronal slices. The user can move throughout the volumes, and create tag points within the volumes. If enough tag points for two volumes are picked, then a transformation is computed for registering the two volumes. This transform is used in displaying the merged image and in allowing the user to start a process which resamples one of the volumes into the same space as the other.