Institutional Review Boards (IRBs) have substantial power and authority over research with human subjects, and in turn, their decisions have substantial implications for those subjects, investigators, and the public at large. However, there is little transparency about IRB processes and decisions. This article provides the first comprehensive taxonomy of what transparency means (or could mean) for IRBs — answering the questions “to whom, about what, and by what mechanisms?” It also explains why the status quo of nontransparency is problematic, and presents arguments for greater transparency from the perspective of a variety of stakeholders. IRB transparency will make boards more accountable, improve the quality of their decision-making, facilitate consistency in board decisions, permit empirical study of IRBs, promote research efficiency, and advance trust in the research enterprise, among a variety of other benefits. Regulators should promote IRB transparency, IRBs themselves should commit to sharing as much information as they can within the confines of confidentiality requirements, and investigators can endeavor to take matters into their own hands by sharing IRB correspondence and IRB-approved protocols and consent materials.