Prostanoids are a group of lipid mediators that include the prostaglandins (PG) and thromboxanes (TX). Upon cell stimulation, prostanoids are synthesized from arachidonic acid via the cyclooxygenase (COX) pathway and released outside the cells to exert various physiological and pathological actions in a variety of tissues and cells. The activities of prostanoids are mediated by specific G protein-coupled receptors, which have been classified on the basis of pharmacological experiments into eight types and subtypes according to their responsiveness to selective agonists and antagonists. These prostanoid receptors have been cloned from various species including human, and their distinct binding properties and signal transduction pathways have been characterized by analyses of cells expressing each receptor. Furthermore, the distribution patterns of prostanoid receptor mRNAs have been determined in tissues and cells for various species. This information is useful for understanding the molecular basis of the pathophysiological actions of prostanoids.