Cyclic GMP is a widely distributed second messenger which was discovered more than 30 years ago. Cyclic GMP is synthesized by soluble and particulate guanylyl cydases and is degraded by the specific phosphodiesterase V. The activity of the guanylyl cydases is controlled by a broad spectrum of substances including nitric oxide (NO), organic nitrates, peptide hormones and toxins. In turn, cGMP regulates various physiological processes ranging from smooth muscle tone, neuronal excitability, epithelial electrolyte transport to phototransduction in the retina. The discovery of a cGMP-dependent protein kinase (cGMP kinase) activity first in lobster muscle (Kuo and Greengard 1970) and then in rat cerebellum (Hofmann and Sold 1972) led to the view that-in analogy to cAMP-dependent protein kinase (cAMP kinase)-cGMP kinase mediates the physiological functions of cGMP. Later it became dear that eukaryotic cells have at least three distinct classes of cGMP receptor proteins: cGMP regulated phosphodiesterases (PDEs)(Sonnenburg and Beavo 1994), cyclic nucleotide gated cation channels (CNGs)(Biel et al. see Chapter 5) and cGMP kinases. Initial biochemical studies and experiments in isolated cells and tissues indicated that cGMP kinase controls physiological functions by similar or identical pathways as cAMP kinases. It was also reported that cGMP kinase is activated by cAMP under physiological conditions suggesting a rather" nonspecific" function of cGMP kinases. More recent experiments which include inactivation of the cGMP kinase genes show that cGMP kinases very specifically regulate distinct cellular functions by pathways which are separate from those used by cAMP kinases. This review summarizes the biochemical, functional and physiological properties of the cGMP kinases.