Theory-based evaluation approaches are becoming increasingly popular in the evaluation of comprehensive community initiatives. Such an approach, Theory of Change (ToC), has been used in the external evaluations of two of four Scottish Health Demonstration projects. This article provides an empirically based analysis of the issues raised in the process of articulating a project's ToC. It then considers the value of the approach in relation to sharpening planning, providing formative feedback, improving performance management, guiding internal and external evaluation, judging impact and reducing problems of attribution. A conclusion is reached that, whilst such approaches are by no means a panacea, they offer one useful framework within which to negotiate a range of evaluation practice decisions. K E Y WO R D S : child health improvement; complex community interventions; heart health improvement; Scottish Health Demonstration projects; theory-based evaluation