This chapter reviews two general approaches to understanding the effects of individual differences on commitment. The trait approach argues for a direct effect of individual differences on commitment and is essentially a between-persons model. The contingency approach argues that the development of commitment from individual differences depends on the situation (that is, a person-by-situation interaction) and can be used to examine both between-persons and within-persons questions about commitment. The contingency approach is argued to be the better framework for researching the relationship between individual differences and commitment. A specific contingency model is reviewed and expanded upon. Practical implications are discussed and three lines of future research on individual differences and commitment are proposed: person-centered (that is, profile) approaches to commitment and individual differences, the effect of individual differences on the stability of commitment, and the effect of individual differences on nested or interrelated commitments.