The encroachment of protected areas for agricultural and livestock production is an important challenge for nature conservation in developing countries. The driving forces of encroachment are debated - major arguments focus on (1) the need of local people to cultivate land inside protected areas due to poverty, (2) commercial interests of cultivating inside protected areas, which indicates free-riding ("greed"), and (3) resistance against protected areas caused by disregard of customary rights. Understanding the role of these factors is important for designing appropriate conservation and development strategies. The paper contributes to this understanding by analyzing the encroachment of a National Park in Sulawesi, Indonesia. The analysis is guided by a theoretical framework which acknowledges that most individuals are neither purely altruistic nor entirely self-interested. The empirical analysis combines data from a village-level survey in 80 villages with information from a sate llite image and other spatial data. The following factors had a significant influence on the extent of encroachment: (1) population density in the area, which is related to needs, (2) the availability of suitable land inside the Park, which indicates "temptation", and (3) the extent of land that was already cultivated before the Park was established, which points to customary rights. Community agreements on conservation are discussed as a policy approach that can address all three factors.