Part I - The theory of stick-slip is developed, based on the concept of a static and dynamic coefficient of friction. The dynamic coefficient is assumed to be independent of displacement and to depend wholly on normal stress across the surface. It is also assumed to be velocity-independent. The theory predicts that the stress drop during stick-slip is independent of machine stiffness. The displacement during stick-slip is by contrast directly proportional to the machine compliance. Two series of experiments with different fault angles would serve to evaluate the dynamic coefficient of friction. Part II - Frictional sliding on sawcuts and faults in laboratory samples of various silicate rocks is markedly temperature-dependent. At pressures from 1 to 5 kb, stick-slip gave way to stable sliding as temperature was increased 200 to 500 C. The particular temperature of transition to stable sliding varied with rock type.