This multi-method study aims to shed light on digital platforms' decisions regarding their openness. Platform openness results from a series of decisions on how open a platform is regarding: (a) suppliers, (b) customers, (c) complementary service providers, as well as to (d) product categories and (e) channels. By conducting a scoping literature review, we analyze the current body of knowledge about the drivers, dimensions and outcomes of platform openness. Using an expert panel discussion and analysis of real-world digital platforms, we confront this existing knowledge with current business challenges to identify research challenges. We address how future research can advance platform research by tackling these challenges.