This research studies the topic of geometric surface trimming. Most of the work that has been done on surface trimming refers to visual trimming, in which, unwanted portions of a surface patch are not removed but simply hidden. Geometric trimming actually removes the unwanted portion of a surface patch, and in the process, produces a new mathematical surface or set of surface patches. The new surface or surface patches produced should closely resemble the original surface which is being trimmed. However, since the entire procedure involves the creation of a new geometry, the resulting surface will not, in general, be identical with the remaining portion of the original one. For this reason, a means to measure the error in approximating the original surface is implemented and this error is minimized. Trimming criteria is provided by mapping the trimming curves from the two-dimensional parametric space on to the three-dimensional B-spline surface. Since visualization for this kind of procedure is vital a tool kit has been developed, using PHIGS, to model B-spline surfaces along with their approximate trimmed surfaces as well as the approximation error involved. / Master of Science