Economic evaluations are increasingly used in decision-making. Accurate measurement of service use is critical to economic evaluation. This qualitative study, based on expert interviews, aims to identify best approaches to service use measurement for child mental health conditions, and to identify problems in current methods. Results suggest considerable agreement on strengths (e.g., availability of accurate instruments to measure service use) and weaknesses, (e.g., lack of unit prices for services outside the health sector) or alternative approaches to service use measurement. Experts also identified some unresolved problems, for example the lack of uniform definitions for some mental health services.