This paper reviews research literature on Diamond Open Access (DOA) journals – sometimes also called Platinum Open Access – that was produced after this journal segment started to become a priority in European research policy around 2020. It con-textualizes the current science policy debate, critically examines different understand-ings of DOA, and reviews studies on the role of such journals in scholarly communica-tion. Most existing research consists of quantitative studies focusing on aspects such as the number of DOA journals, their publication output, the diversity of the landscape in terms of subject areas, languages, publishing entities, indexing in major databases, awareness and perception among scholars, cost analyses, as well as insights into the internal operations of DOA journals. The review shows that research on DOA journals is partly influenced by the science policy discourse in at least two ways: first, through the normativity inherent in that discourse, and second, through the temporality of policy-driven research of practical relevance, which leaves important aspects of the phenomenon understudied. Moreover, research on the DOA journal landscape has implications beyond understanding this particular journal segment, as it also challenges established views of the global system of scholarly communication.