This introductory lecture will focus on the main research traditions in non-profit sector studies. It centres on an institutional choice approach which can be divided into two blocs; the failure performance approach explaining the existence of civil society organizations – understood here primarily as non-profit organizations – as a failure of other institutions, and the transaction cost approach, which explains the institutional forms with alternative contractual arrangements. Within this framework, the most influential theories will be examined: public goods theory (Weisbrod), heterogeneity theory (James), trustworthiness theory (Hansmann, Arrow, Nelson & Krashinsky), third party government theory (Salamon) as representatives of the former stream, and the concept of transaction costs (Williamson, Krashinsky, Ben-Ner & van Hoomissen) as representative of the latter.