The problem of defining and measuring contractility and inotropy of the cardiac sarcomere and of the whole heart are discussed. The mechanical techniques for deducing and controlling sarcomere lengths and the use of electron microscopy for measuring sarcomere lengths in chemically fixed preparations are compared with recent studies in which the lengths were measured directly with light microscopy of light diffraction. The consequences of muscle fiber buckling and sarcomere length inhomogeneity on the mechanical properties of the muscle are discussed. The relationship between muscle tension, passive and active sarcomere lengths, and the inotropic state of the muscle are reviewed. Possible explanations for the opposing views as to whether muscle length is an important determinant of the inotropic state of the muscle are considered. Isovolumic and ejection phase indices based on assumed and observed mechanical characteristics of the heart have been used to evaluate contractility. The validity of th assumptions and the advantages and disadvantages of each index are discussed.