Base rate information is important in clinical assessment because one cannot know how unusual or typical a phenomenon is without first knowing its base rate in the population. This study empirically determined the base rates of social skills acquisition and performance deficits, social skills strengths, and problem behaviors using a nationally representative sample of children and adolescent ages 3-18 years. Using the national standardization sample of the Social Skills Improvement System-Rating Scales (N = 4,550) across 3 informants (teacher, parent, and student) and across 3 broad age groupings (3-5 years, 5-12 years, and 13-18 years), these base rates were computed. Results showed that the base rates for social skills acquisition deficits and problem behaviors are extremely low in the general population. Base rates for social skills performance deficits and social skills strengths were considerably higher, with students in the 5- to 12-year-old age group reporting fewer performance deficits and more social skills strengths than older children (13-18 years). Teachers and parents reported more performance deficits and fewer social skills strengths across all age groups than students in the 5- to 12-year-old age group. These results are discussed in terms of the utility of base rate information in clinical decision making. (Contains 5 tables.)