Current modeling and segmentation techniques are not adequate for visualizing certain kinds of 3D medical data. This paper introduces volumetric deformable models, or active blobs, as a means of visualizing and segmenting 3D data. It presents an implicit framework that provides a means of generalizing deformable models to higher dimensions. It presents a new second- order smoothing function that offers some advantages over previous methods for smoothing surfaces. It describes some of the numerical techniques that are necessary for solving the resulting evolution equations. Finally it shows some examples of how active blobs are useful for the segmentation and visualization of 3D ultrasound and MRI data. Active blobs have a number of advantages over other types of modeling techniques. Because of these advantages, active blobs are able to produce models directly from the 3D data, rather than presegmented datasets, edge maps, or sets of landmarks.