The concept of active nucleation sites is examined in the light of standard nucleation theory and it is suggested that these sites are probably small re-entrant corners or jogs in growth steps on the surface of the nucleating particle. The nucleation behavior of a spherical particle with a conical pit on its surface is examined in detail and the activity of a population of particles having a log-normal distribution of such pits is worked out. The parameters involved in the theory can be evaluated within acceptable limits and the resulting curves exhibit the phenomena observed experimentally. Generalized activity curves are given in parametric form for more detailed comparison with experiment. Abstract The concept of active nucleation sites is examined in the light of standard nucleation theory and it is suggested that these sites are probably small re-entrant corners or jogs in growth steps on the surface of the nucleating particle. The nucleation behavior of a spherical particle with a conical pit on its surface is examined in detail and the activity of a population of particles having a log-normal distribution of such pits is worked out. The parameters involved in the theory can be evaluated within acceptable limits and the resulting curves exhibit the phenomena observed experimentally. Generalized activity curves are given in parametric form for more detailed comparison with experiment.