Scaling suggests providing a real solution for large numbers of people. Public–private partnerships (PPPs) are seen as an important way to reach scale, as they combine the competencies of different actors to address difficult development issues, create breakthroughs, and combine different types of finance to create and sustain solutions. Scaling is an increasingly popular concept. This popularity, however, is not matched by a high level of conceptual clarity or depth of approaches. The main aim of this booklet is to unpack the concept of scaling and make it more understandable and actionable. A second focus is on the role of public–private engagement in making scaling work.