Research suggests that attachment patterns can be measured as early as 12 months of age and that in the absence of major environmental change, they persist into adulthood. Evolving from three traditions, a number of assessment tools have been developed to study adult attachment. They range from semiclinical interviews to brief questionnaires and explore different relationship domains, from retrospective accounts of childhood experience to current romantic relationships. After exploring the theoretical background of the study of adult attachment, the authors review the format and the psychometric properties of several measures. Studies that compare measures are described. The article concludes with a discussion of unresolved questions about adult attachment that emerge from various measurement perspectives.