The essential light chain of myosin (ELC) is known to be important for structural stability of the α-helical lever arm domain of the myosin head, but its function in striated muscle contraction is poorly understood. Two ELC isoforms are expressed in fast skeletal muscle, a long isoform and its N-terminal ~40 amino acid shorter counterpart, while only the long ELC is observed in the heart. Biochemical and structural studies revealed that the N-terminus of the long ELC can make direct contacts with actin but the effects of the ELC on the affinity of myosin for actin, ATPase, force, and the kinetics of force generating myosin cross-bridges are inconclusive. Myosin containing the long ELC has been shown to have slower cross-bridge kinetics than myosin with the short isoform. A difference was also reported among myosins with long isoforms. Increased shortening velocity was observed in atrial compared to ventricular muscle fibers. The common findings suggest that ELC provides the fine-tuning of the myosin motor function, which is regulated in an isoform and tissue dependent manner. The functional importance of the ELC is further implicated by the discovery of ELC mutations associated with Familial Hypertrophic Cardiomyopathy. The pathological phenotypes vary in severity, but more notably, almost all ELC mutations result in sudden cardiac death at a young age. This review summarizes the functional roles of striated muscle ELC in normal healthy muscle and in disease. Transgenic animal models and phenotypic characterization of ELC-mediated remodeling of the heart are also discussed.