This chapter addresses the question of to what extent a secure attachment relationship during infancy is related to mastery motivation in toddler-hood. It explores the relationship between different aspects of mastery behaviour displayed within the same setting. In addition, a considerable number of studies have focused upon the effects of children's experiences on the development of their motivation to explore and master the environment. With regard to the effects of experience within infancy, research has been conducted from two different theoretical perspectives, suggesting different mechanisms to explain and specify the effects of experience upon the development of mastery motivation. The conceptual framework of attachment theory offers a different explanation for the effects of early experience upon a child's motivation to explore and master the environment. In attachment theory, mastery motivation views as being under the control of the exploratory behavioural system. The working of the exploratory system is presumed to be inversely related to activation of the attachment behavioural system.