Fibrillar collagen forms the major structural component of the cardiac extracellular matrix. Whereas the central contribution of collagen to the mechanical properties of many connective tissues is well known, the specific role of the cardiac collagen matrix in ventricular mechanics remains unclear and controversial. A review of the cardiac extracellular matrix and the mechanical effects of altered cardiac collagen structure and content suggests that collagen in the myocardium may affect ventricular mechanical function significantly. Mathematical models show that the functional effects of changes in the cardiac collagen matrix during disease probably depend on specific, measurable microstructural parameters.