In this article we measure, describe, and demonstrate the importance of differential treatment for developing countries in multilateral environmental agreements (MEAs). So far, we argue, quantitative research on differentiation has been minimal due to data constraints and the complex nature of relevant provisions. In response, we offer a way of relieving this constraint, exploiting the fact that MEAs with differentiation typically identify distinct sets of “developing country” parties. After describing the data collection process, we show that differentiation is surprisingly uncommon, appearing in only 6 percent of MEAs, and disproportionately appears in larger, more recent agreements. We then test a key conjecture about differentiation by revisiting the debate on the depth–participation dilemma. We demonstrate, specifically, how it conditions this relationship. When MEAs do not differentiate, greater depth reduces participation; when they do, the relationship is reversed, making it possible to sustain high levels of both. This result helps to reconcile conflicting findings in earlier studies and has important policy implications.