Therapeutic alliance constitutes a major variable in explaining the outcome of treatment. However, meta-analytic, narrative, and theoretical reviews have all begun to comment on significant deficiencies in both conceptualization and measurement of alliance. While the evidence on the overall impact of alliance on treatment outcomes is impressive, we know much less about its components, modelling and discrete measurement. We review the conceptual and methodological underpinning of current alliance concepts and measures with the aim of identifying the current status of the methods and clarifying the conceptual and measurement tasks ahead. The review makes clear the diversity of concepts and measures available to address treatment alliance; however there is no one current measure of alliance that meets all the predefined criteria in either adult or child populations. We discuss the most successful measures to date, and recommend future developments in the field of alliance, including conceptual and experimental approaches, developmental issues, and analytic techniques.