Rho of plants (ROP) proteins, also known as RAC proteins, are Rho-related GTPases that function as molecular switches in a multitude of signaling cascades involved in the regulation of the actin and microtubule cytoskeleton, of vesicle trafficking, and of plant responses to hormones, stresses, or light (Yang, 2002; Berken, 2006; Nibau et al., 2006; Yang and Fu, 2007). Rho GTPases are Ras-related small guanine nucleotide-binding proteins (G-proteins) that bind GTP and GDP with high affinity and hydrolyze GTP inefficiently. Rho GTPases switch between GTP-on to GDP-off states by highly regulated GDP/GTP exchange and GTP hydrolysis (Bourne et al., 1991; Vetter and Wittinghofer, 2001). Only in the GTP-bound state can Rho GTPases interact with effectors to elicit downstream signaling. The GDP/GTP exchange is catalyzed by guanyl nucleotide exchange factors (GEFs), and GTP hydrolysis is enhanced by GTPaseactivating proteins (GAPs). Like other members of the Ras superfamily of small G-proteins, Rho GTPases are soluble proteins that associate with and function at cell membranes by virtue of the posttranslational lipid modifications prenylation and S-acylation (Hancock et al., 1989; Michaelson et al., 2001). A third group of regulating proteins are Rho guanyl nucleotide dissociation inhibitors (RhoGDIs), which inhibit GDP/GTP exchange and facilitate the cycling of Rho GTPases on and off membranes (DerMardirossian and Bokoch, 2005). The ability of Rho GTPases to interact with membranes allows these proteins to regulate actin polymerization and vesicle trafficking at discrete sites of the plasma membrane and of internal membranes, which is essential for their role in the control of cell polarity (Ridley, 2006). As part of the Plant Physiology focus issue on membrane biology, this review focuses on subcellular targeting of plant ROP/RAC GTPases and on the role of these proteins in the regulation of membrane trafficking, cytoskeleton organization, and cell polarity. Other aspects of ROP/RAC biology, such as the role of these GTPases in hormonal or stress signaling, will only be summarized in brief. We refer interested readers to several excellent recent reviews on ROP/RAC GTPases that highlight these other topics (Molendijk et al., 2004; Xu and Scheres, 2005; Nibau et al., 2006; Yang and Fu, 2007; Berken and Wittinghofer, 2008; Kost, 2008). Throughout this review, we have opted to use the ROP nomenclature for the sake of clarity. However, we use the RAC terminology in instances in which there is no ROP nomenclature or when publications have used the term RAC rather than ROP. Finally, we apologize to those colleagues whose work we have not been able to cite due to lack of space.