Bowlby's concept of mental working models of self, attachment figures, and the social world has been theoretically generative as a bridge between early relational experience and the beliefs and expectations that color later relationships. Contemporary attachment researchers, following his example, are applying new knowledge of children's conceptual development to their study of attachment-related mental representations in children and adults. The contributors to this special issue highlight recent advances in how the mental representations arising from attachment security should be conceptualized and studied, and identify a number of important directions for future work. This paper introduces the special issue by summarizing the major ideas of Bowlby and his followers concerning the nature and development of mental working models, points of theoretical clarity and uncertainty, and challenges in assessing these representations, as well as profiling each of the contributions to this issue.