In 2000, Carliss Baldwin and Kim Clark published “Design Rules: The Power of Modularity,” a book that introduced new ways of understanding and explaining the architecture of complex systems This Special Issue of Industrial and Corporate Change celebrates this seminal work, the research it has inspired, and the insights that these collective efforts have generated. In this introductory essay, we review the impact of “Design Rules” across numerous fields, including organization theory, competitive strategy, industry structure, and innovation management. We offer perspectives on key themes that emerge from contributions in this issue, including the alignment between organizational and technical designs (“mirroring”), the dynamics of industry evolution, and the role that individuals play in shaping and responding to system designs. We close by highlighting opportunities to apply the theory in Design Rules to new phenomena and puzzles that have emerged in the past 20 years.