Cardiomyopathies are diseases of heart muscle that are associated with cardiac dysfunction. Molecular genetic studies performed to date have demonstrated that the damage or mutations in several sarcomeric contractile protein genes are associated with the development of the diseases. In this review, cardiac troponin I, one of the sarcomeric thin filament protein, will be discussed regarding its role in cardiac function, its deficiency-related diastolic dysfunction, and the mutation of this protein-mediated restrictive cardiomyopathy.