The hydrolysis of guanosine triphosphate (GTP) is a key process in numerous vital processes. Intracellular signal transduction, protein synthesis, vesicular and nucleocytoplasmic transport, protein targeting, growth control and differentiation, are all, among other processes, controlled enzymatically by the conversion of GTP into GDP and inorganic phosphate. GTPases are the molecular switches that catalyze this reaction. They cycle between two conformational states: one bound to GTP (\'active\' state), the other bound to GDP (\'inactive\' state), and they hydrolyze GTP to GDPand inorganic phosphate. In the \'on\' (GTP) state, GTPases recognize target proteins and generate a response until GTP hydrolysis returns the switch to the \'off\' state. GTPases can be classified into six families of heterotrimeric G-protein, small monomeric, protein-synthesizing, signal-recognition-particle, dynamin, and tubulin GTPases (EC 3.6.5.1 – EC 3.6.5.6 respectively). GTP-hydrolysis by GTPases is intrinsically very slow but can be accelerated by orders of magnitude upon interaction with GTPase-activating proteins (GAPs). In order to bring the GTPase back in the GTP bound ‘on’ state, so called guanine nucleotide exchange factors (GEFs), which cause the GDP to dissociate from the GTPase, leading to its association with new GTP.