This chapter describes three studies that shed light on: the validity of conceptualization of mastery motivation, developmental changes in such motivation during infancy and toddler-hood. It examines the relation between task persistence and task pleasure, indices of two central aspects of mastery motivation. The chapter explores correlates of persistence and pleasure, to help provide construct validation for them as measures of mastery motivation. It examines continuities and changes in behaviour, to provide evidence regarding the development of mastery motivation between 6 and 30 months of age. The results of the three studies provide support for the conceptualization of mastery motivation outlined in the introduction and for the proposed transitions in mastery motivation during the latter part of the first and second years of life. The chapter helps to stimulate research aimed at better determining the nature and implications of individual differences in mastery motivation during the first three years of life.