This article examines the structural effects of platform certification on the supply of complementary products. Drawing on the exploratory case of mobile application markets, the article highlights the broader market effects of competing platforms and their certifications on a platform-based complementary product market. The case suggests that platform certifications influence market intermediation, entry barriers, and deployment fragmentation. We present these market effects in a conceptual model that can be applied to understand similar complementary product markets. As such, the article contributes to the literature on compatibility standards by emphasizing some of the complementary product market effects of employing certification in enhancing compatibility.