Research in developmental psychopathology has long been preoccupied with rather broad categories of behavior, but we know little about the specific behaviors that comprise these categories. The objective of this study was to: (a) estimate the prevalence of problem and social competence behaviors in the general population of children at 17 months of age, and (b) describe the continuity and discontinuity in the degree to which children exhibit these behaviors between 17 and 29 months of age. The results show that frequent problem behaviors are not typical of children under two years of age. Further, the results suggest that it is possible to distinguish between different types of problem behaviors before two years of age. In addition, the results show that gender differences in some problem behaviors are already present before two years of age, and increase in magnitude during toddlerhood. Finally, the results show that interindividual differences in problem behaviors observed before two years of age are stable. The predictive accuracy of frequent problem behaviors in children at 17 months of age was limited, however, with often a majority of toddlers not behaving this way a year later. Overall, our results suggest that toddlerhood represents a critical period when behavioral and emotional problems of potentially clinical significance emerge. Pediatricians should routinely ask parents to report the frequency of their young children's problem behaviors during child health supervision visits so that children whose frequent problem behaviors persist over time can be identified and possibly referred for treatment.