What are we trying to index when we make assessments of attachment patterns in early life? How do these quality-of-attachment constructs relate to other constructs that are theoretically important to the description and development of attachment relationships? How do parents and children contribute to these developmental processes? What measurement issues are important for the field to consider when evaluating attachment and related constructs? In addressing these questions, we will concurrently examine the nature and assessment of constructs of maternal sensitivity and temperament, both of which have been significant in attachment research and theory. We begin by describing the core constructs of attachment theory, namely, the attachment system and secure-base behavior. We follow by discussing contextual factors that are thought to be crucial in the development of individual differences in attachment, especially maternal sensitivity. We then consider child characteristics, especially temperament, that may contribute to the operation of the attachment system. In this context, we consider both parental supports for secure-base behavior and the ways by which infants teach their parents about the specific behaviors that are required to provide a secure base for the individual child. Assessment issues associated with each of these constructs, connections (theoretical and empirical) among them, and some views on directions for future research are each discussed in turn.