Payments for environmental services (PES) have attracted increasing interest as a mechanism to translate external, non-market values of the environment into real financial incentives for local actors to provide environmental services (ES). In this introductory paper, we set the stage for the rest of this Special Issue of Ecological Economics by reviewing the main issues arising in PES design and implementation and discussing these in the light of environmental economics. We start with a discussion of PES definition and scope. We proceed to review some of the principal dimensions and design characteristics of PES programs and then analyze how PES compares to alternative policy instruments. Finally, we examine in detail two important aspects of PES programs: their effectiveness and their distributional implications.