Myosin, a molecular motor, converts chemical energy into mechanical force. The motor domain of myosin heavy chain (MyHC) includes an ATP binding region with ATPase activity and an actin-binding region. Motor function is achieved by conformational changes, at hydrolysis, of ATP causing a shift in the angle between the actin binding head and the rod region of the molecule. The elongated alpha-helical coiled-coil rod region of MyHC molecules constitutes the major part of the thick filaments of the sarcomere. Three major MyHC isoforms are expressed in human skeletal muscle (type I, MYH7, expressed in type 1 fibres; IIa, MYH2, expressed in 2A fibres; IIx, MYH1, expressed in 2B fibres). While mutations in slow/beta cardiac MyHC (MYH7) are a common cause of familial hypertrophic cardiomyopathy, no skeletal myopathies have, until recently, been associated with mutations in MyHC. A heterozygous mutation, Glu706Lys, in the core of the head of MyHC IIa is associated with a familial congenital myopathy, which, in most instances, has shown mild phenotypic expression in children but progressive course in some adults. There is a relationship between the level of expression of mutated MyHC IIa and muscle pathology. Some adults with a progressive course show muscle fibres with rimmed vacuoles and filaments of the type seen in inclusion body myositis/myopathy (IBM). Endurance training in a group of affected patients caused a shift in the expression of myosin from fast (IIx) to slow (I) isoforms but no reduction in the expression of MyHC IIa. A heterozygous mutation, Arg1845Trp, in the distal rod region of slow myosin (type I, MYH7) is associated with familial congenital myopathy, with large deposits of MyHC I in the subsarcolemmal region of type 1 muscle fibres, "Myosin storage myopathy". These patients showed slowly progressive muscle weakness but no overt cardiomyopathy. These two muscle diseases, which are caused by mutations in MyHC, form the basis of a novel entity: "Myosin myopathies".