The core infrastructure of Globus Toolkit 3 (GT3 Core) is based on the Open Grid Services Infrastructure (OGSI) primitives and protocols. The main design goal has been to make the OGSI technology easy to use, reuse, and extend when developing new Grid applications. In addition to an open source implementation of all the OGSI defined protocols, which can be used as a reference for other OGSI implementations, GT3 Core also provides various hosting environments built around a container abstraction. The container enables portable OGSI compliant Grid services to be developed without any knowledge of the underlying protocols and transport bindings. Our implementation has been built on top of, and hence leverages, state-of-the-art Web services technology. GT3 Core can be seen as the set of building blocks we consider essential for all Grid applications. The OGSI primitives implemented offer support for soft-state management, inspection, notification, discovery and global instance naming. Additionally, GT3 Core is comprised of a security infrastructure, and a number of system-level services, such as logging-, management-, and administration Grid services. Furthermore, a development environment with code generation tools to simplify and accelerate development of new services is included.