During the last few years there has been growing interest in the large family of GTPases structurally related to protooncogene product p21 ras. These proteins share structural similarities but are involved in a large spectrum of biological functions. While members of the ras subfamilyregulate cell growth, rho proteins are implicated in the control of cytoskeletal organization in mammalian cells or in bud formation in yeast. Members of the ARF/SAR family are postulated to control vasicle budding, whereas Ypt1/Sec4/rab proteins are thought to regulate vesicle targeting and/orfusion. Owing to their functional divergence, the small GTPases have attracted the attention of many investigators in the fields of structural biology, cell biology, cell growth and differentiation and developmental biology. Numerous studies have contributed important data on the biochemicalfeatures, the intracellular focalisation, and in some cases, the function of these proteins. This book contains entries on almost 150 GTPases which have been identified so far, and the search has not yet been exhausted. This large repertoire of information is gathered together in an easy toconsult format. This book is meant to provide a catalogue of the information currently at hand to established scientists in the field, as well as to introduce more investigators to their proteins of interest.