Selection intensities (i) appropriate to pig breeding operations were examined in relation to variations in structure and size of herd, level of testing, and generation interval. Values of i for boars were larger than for sows and responded in greater degree to the changes in population size associated with variations in herd size and number of pigs surviving to breeding age in each litter. In closed herds with a boar sow ratio of 1 : 5, relatively little improvement in i resulted from increasing the size of breeding unit beyond 4 boars and 20 sows. Increasing the number of sows bred to each boar resulted in a substantial improvement in selection intensity. The reduction in litter size associated with litter testing for carcass evaluation decreased population size and reduced the opportunity for selection. Application of these findings in relation to herd and breed improvement is discussed.