The study of mother-infant attachment came of age during the 1970s and has now broadened its influence in psychology well beyond the period of infancy. British clinician John Bowlby set the stage for this advance with an ethological model of attachment incorporating ideas from ethology, primatology, object relations theory, and systems theory. His theoretical synthesis was complemented by methodological and empirical advances by Mary Ainsworth, Alan Sroufe, Everett Waters, Mary Main, Jay Belsky, and many others. In this chapter, research during infancy based upon Ainsworth’s original strange situation paradigm is described as well as contemporary longitudinal research that examines the developmental significance of early attachment over the life course. Finally, attachment patterns are interpreted from a life history perspective as resourcedependent reproductive strategies that are set in motion in infancy.