Computational science portals are emerging as useful and necessary interfaces for performing operations on the Grid. The Grid Portal Development Kit (GPDK) facilitates the development of Grid portals and provides several key reusable components for accessing various Grid services. A Grid Portal provides a customizable interface allowing scientists to perform a variety of Grid operations including remote program submission, file staging, and querying of information services from a single, secure gateway. The Grid Portal Development Kit leverages off existing Globus/Grid middleware infrastructure as well as commodity web technology including Java Server Pages and servlets. We present the design and architecture of GPDK as well as a discussion on the portal building capabilities of GPDK allowing application developers to build customized portals more effectively by reusing common core services provided by GPDK.