Partnering for Change (P4C) is an innovative practice model for school-based occupational therapy developed in Canada and informed by a program of research spanning nearly two decades. National and international interest in P4C necessitated development of an explanatory theory to guide implementation in varied contexts. The purpose of this study is to document the process of theory development and to provide an overview of the initial P4C explanatory theory. Realist evaluation was used to construct an initial explanatory theory of P4C drawing on P4C developers' conceptualisations of the model, document analysis, and analysis of transcripts from interviews and focus groups. No consumers were involved in the study design or analysis. Four sets of premises expressed as context-mechanism-outcome configurations (CMOCs) comprise the initial explanatory theory of P4C. An abstracted summary of these four sets along with contexts, mechanisms, and outcomes central to the P4C initial explanatory theory is presented. An exemplar CMOC is shared to illustrate how theoretical premises are developed and expressed using realist evaluation. This study advances knowledge regarding the hypothesised core elements of P4C and provides an example of using realist evaluation to advance knowledge in occupational therapy. Preliminary implications for clinical practice are discussed. Partnering for Change (P4C) is a way of providing occupational therapy services in schools that helps all children to learn and participate successfully. P4C was created in Ontario, Canada, and has been researched for almost 20 years. People in different parts of Canada and other countries are interested in trying P4C. As the researchers who created P4C, we think there is a need to explain how it works so it can be used in different places. This study explains how we created a theory to describe P4C. To develop this theory, we used a method called realist evaluation that would help us explain how, why, and in what situations P4C works best. To do this, we analysed documents and transcripts of interviews and focus groups. Our findings are organised into four main ideas, each explained in terms of contexts (the situations), mechanisms (how and why P4C works), and outcomes (the impacts of P4C). One detailed example is given to show how these ideas were formed and work together. In conclusion, our study helps us understand the key parts of P4C and shows how realist evaluation can be used to improve knowledge in occupational therapy. The study also suggests some early ideas on how P4C can be used in practice.