Two studies investigated whether situations are associated with the manifestation of Big Five trait contents in behavior. Several times per day for 2 or 5 weeks, participants reported their current Extraversion, Agreeableness, Conscientiousness, and Emotional Stability states and rated the concurrent situation on several characteristics. Multilevel models tested for the average individual's contingency of each Big Five state on each situation characteristic and for whether individuals differed from each other reliably in those contingencies. Results showed that (1) there are psychologically active characteristics of situations on which trait-manifesting behavior is contingent; (2) contingencies on psychologically active characteristics of varying situations are part of the explanation for the sizeable within-person variability in behavior; (3) individuals differ reliably in their contingencies, and such individual differences may partially explain individual differences in amount of variability; and (4) the situation characteristics that are psychologically active differ by trait. These findings suggest that within-person variability in personality states is meaningful and is related to situations, that personality psychology should characterize situations in terms of their relevance to personality states, and that process and individual-difference structure approaches can be integrated in personality psychology.