In the last 10 years researchers have firmly established key roles for R- related GTPases in almost every aspect of cell biology. In the 1980s the pro- oncogene Ras itself was the focus of interest, though in the 1990s this shifted to the increasing variety of Ras-related proteins. In this new decade much yet needs to be done to establish the role for all the small GTPases now uncovered by the human genome project. In particular, these GTPases need to be und- stood in the appropriate biochemical and cellular contexts. In the process of trying to uncover the role of these versatile proteins, a variety of novel te- niques and methodologies has been developed. These now enable investi- tors to move easily within a diversity of fields ranging from structural studies to real-time in vivo analysis of a GTPase. In recognition of the need for access to key background methodologies, GTPase Protocols: The Ras Superfamily is devoted to techniques that are pr- ently widely used and that will continue to be the standard for researchers worldwide. Each chapter is aimed at supplying detailed methodologies to allow reproduction in any laboratory, while also providing the general pr- ciples on which the methods are based. Some of the techniques grouped in the first section apply broadly to small GTPases, whereas others in Part II are more applicable within each GTPase subfamily.