It is becoming increasingly evident that an understanding of incipient microcracking and growth of small cracks is essential to the development of improved predictions of the fatigue life of structures. Information on the threshold and kinetic properties of small cracks is reviewed and critically discussed. It is shown that the use of conventional fracture mechanics concepts to characterize small cracks results in behavior which differs from that of large cracks—this difference is due to a breakdown of underlying continuum mechanics assumptions. Methods to incorporate small crack behavior in fatigue life predictions are also considered. In these predictions, the importance of separately treating crack initiation and crack growth and of accounting for small crack behavior and plasticity effects (particularly for notched members) is demonstrated.