Intelligence agencies routinely use surveillance technology to perform surveillance on digital data. This practice raises many questions that feed a societal debate, including whether the surveillance technology is effective in achieving the given security goal, whether it is cost-efficient, and whether it is proportionate. Oversight bodies are important actors in this debate, overseeing budgets, legal and privacy matters, and the performance of intelligence agencies. This paper examines how oversight bodies evaluate the questions above, using documents produced by American and British oversight mechanisms.