Studies of the interaction of wolves and their prey during the past twenty years have demonstrated that ungulates are the primary prey of wolves both in winter and summer and that predation in summer tends to be concentrated on the young of the year while in winter it is concentrated on animals in older-age classes. There appears to be intrinsic control of wolf numbers and there is evidence which suggests that a wolf per 10 square miles may approach the maximum density that is attainable in most ranges. The adaptations between ungulates and their predators may have evolved in relatively stable forest environments which could not support high-density prey populations. This could explain why wolves do not appear to be capable of controlling moose and deer populations in environments that have been drastically altered by man.