The migration of an existing application from UNIX to Windows NT is a relatively small step for the developer and one that can have giant benefits to users. I managed and participated in the migration of two large ArcTools applications from UNIX to Windows NT. These applications are used to maintain the San Diego City and County SanGIS data (formerly RUIS data). The data that is maintained is made up of over a million polygons that are stored in ArcStorm. This paper will cover the following: 1. Process of the migration 2. Challenges of the migration 3. The advantages/disadvantages in Windows NT