This paper provides a methodology for categorizing public-private partnerships in infrastructure, based on the following key characteristics: whether the project involves new or existing business, the nature of the private sector's construction obligations, the need for the private sector to mobilize significant private funding ab initio, the nature of the private sector's service delivery obligations, and the source of the project revenue stream. The purpose of this methodology is to facilitate mapping, referencing, cross-comparison, analytical studies, and descriptions of public-private partnerships in infrastructure projects with similar key characteristics across sector, commercial, regional, and geopolitical lines. The methodology is tested against 15 case studies representing different infrastructure sectors, regional applications, and commercial approaches to public-private partnerships.