Mastery motivation is the psychological force to achieve and improve one's skills in the absence of any physical reward. This article presents the theoretical background for this concept and describes three characteristics of mastery motivation in toddlers: (1) behavioral manifestations in multiple domains, (2) developmental changes in those manifestations, and (3) the influence of the socializing environment. These characteristics are used to suggest applications for assessment of individual differences (measurement) and for program design (intervention)