© 2018 Elsevier B.V. Research on Payments for Environmental Services has only recently started to pay attention to motivation “crowding”, i.e. the effect that such rewards might have on either strengthening (crowding-in) or weakening (crowding-out) participants' intrinsic motivations to protect and sustainably manage natural ecosystems. In this Introduction to the special issue Crowding-out or crowding-in? Behavioral and motivational responses to economic incentives for conservation, we propose a conceptual framework that maps out how PES implementation, or incentive-based conservation more broadly, might lead to motivation and behavioural change, drawing on theoretical insights and empirical evidence from behavioural economics and social psychology. We also explain how PES design and implementation factors, such as payment type, communication and verbal rewards, inclusive and participatory decision-making, and monitoring and sanctioning procedures, might harm or enhance intrinsic motivations. We suggest that motivation crowding depends on how these policy features are perceived by and affect an individual's need for satisfaction, modulated in turn by the stimulation or inhibition of competence, autonomy, social and environmental relatedness. We highlight the importance of measuring these variables and their motivation and behavioural outcomes in future PES research, in order to relate psychological processes with other contextual determinants of PES social-ecological performance.