Signal processing via heterotrimeric G-proteins is one of the most widely used systems for signal transfer across the cell membrane. This signaling system regulates most physiological and pathophysiological processes in mammals and is therefore the primary target of many pharmaceutical agents. The heterotrimeric G-protein signaling system includes the G-protein-coupled receptor (GPCR), heterotrimeric G-proteins, and effectors. The G-proteins are activated by the GPCR to mediate a signal to effector molecules. However, other players in this system that regulate the activation status of heterotrimeric G-proteins independently of GPCR have been identified. Such accessory proteins for heterotrimeric G-protein can provide additional signal input to the G-protein signaling system, or may act as alternative binding partners of G-protein subunits serving as yet unknown roles in cells. It has been reported that this class of proteins is expressed in the cardiovascular system and contributes to signal integration involved in the various diseases. This review provides an overview of the current understanding of accessory proteins for heterotrimeric G-proteins in their 4 functional subsets, including guanine nucleotide exchange factors (GEFs), guanine nucleotide dissociation inhibitors (GDIs), GTPase-activating proteins (GAPs), and Gβγ-interacting proteins, and discusses their roles in the development of cardiovascular diseases. Better understanding of these components may contribute new insight into the complex network of molecules governing GPCR signaling in the cardiovascular system.  (Circ J 2013; 77: 2455–2461)