Students' active role is heavily emphasised in contemporary assessment research, policies and practices. Even then, students remain the objects of assessment in an era defined by accountability measures, testing and measurement: students are rarely heard on why, how and when assessment is conducted. In this way, assessment may hinder educational systems in promoting ethics and democratic values. In this study, we explore the idea of students as partners in assessment (SaPA) to address these fundamental issues of assessment. In SaPA, students may get their voices heard in shaping assessment design and policy. We conduct a realist review of SaPA literature to clarify the conceptualisation of SaPA and to synthesise the fragmented pieces of empirical evidence. In the spirit of realist reviews, we synthesise the reported SaPA designs and their outcomes by examining what works, for whom, why, and under what circumstances. Our analysis considers 46 studies published between 1994-2022. We formulate three theories of SaPA to guide future research and practice: (1) The outcomes of SaPA are uncertain but can be optimised, (2) SaPA is a relational practice, and (3) SaPA promotes democratic values. We provide an organising framework to guide research and practice, calling for conceptual clarity and methodological rigour in future work. We conclude that in an era of measurement, SaPA provides powerful yet risky affordances for seeing students as meaningful agents in assessment design, implementation, policy and evaluation.