Hypercontraction or abnormal contraction of vascular smooth muscle is a major cause of diseases such as hypertension and vasospasm of the coronary and cerebral arteries. A better understanding of the mechanism of regulation of smooth muscle contraction should lead to improved treatments for such diseases. Recent studies have revealed important roles for the small GTPase Rho and its effector, Rho-associated kinase (Rho kinase) in Ca²⁺-independent regulation of smooth muscle contraction. The Rho–Rho-kinase pathway modulates the level of phosphorylation of the myosin light chain of myosin II, mainly through inhibition of myosin phosphatase, and contributes to agonist-induced Ca²⁺-sensitization in smooth muscle contraction. Rho–Rho-kinase mechanisms also participate in a variety of the cellular functions of non-muscle cells, such as stress-fibre formation, cytokinesis and cell migration. This review summarizes the role of the Rho–Rho-kinase pathway in contractile processes of smooth muscle and in non-muscle cell functions, and the pathophysiological implications of this pathway.