Since its original conception by Bowlby to explain an important evolutionary function of the childcaregiver relationship, attachment theory has been supported by much empirical research in a variety of settings. However, although attachment theory began as a clinical enterprise, its wider application to the everyday clinical understanding of adult mental health problems has lagged behind the available research. The theory can afford valuable insight not only into the developmental nature of common psychiatric disorders, but also into the development of the therapeutic relationship. This article gives an overview of (a) the current conceptualisations of attachment and the measurement of attachment for clinical research purposes and (b) the application of attachment theory to different psychopathologies. The importance of attachment to the therapeutic relationship will be discussed in a future article.Ken Ma is a Specialist Registrar in Child and Adolescent Psychiatry at The Blakesley Centre (102 Blakesley Road, Birmingham B25 8RN, UK. Email: kenkkma@yahoo.co.uk), and is also an Associate Clinical Teacher at the University of Warwick. His interests include the contribution of attachment theory to personality development and the help-seeking literature, and the assessment of attachment in adolescents and adulthood.