There is a great deal of confusion about the meaning of the concept public-private partnership (PPP). Much is written on the subject but only rarely do authors give an adequate account of what they mean when they talk about PPP, nor do they acknowledge that there exist qualitatively different PPP research traditions. The lack of conceptual clarity leads to false statements about PPP, since what is true for one specific PPP-type does not necessarily apply to other forms of PPP. This paper reviews the literature on PPP and argues that there exist at least five distinct approaches in the burgeoning PPP literature with different origins and that it is vital for analytical clarity that authors place themselves within one of these traditions when doing research on PPP.