Siblings, who are 50% similar genetically and grow up within the same family, nevertheless differ markedly in personality and psychopathology, and most of these sibling differences cannot be explained by genetic factors. These findings from the field of behavioral genetics imply that within-family processes that lead to sibling differences, called nonshared environment, are crucial for understanding environmental influences on individual development. Such nonshared environmental influences cannot be identified by the conventional strategy of comparing one child per family on a family-by-family basis; what is needed are studies of siblings that focus on why they are so different. The implications of these findings for investigating family process are outlined, and research is reviewed that explores the extent to which siblings in a family have different experiences, and that begins to assess links between such differential experiences and developmental outcomes.