Project portfolio management (PPM) has since the 1950s offered principles helping us to act in PPM-related situations. Our study integrates what we know by reviewing seven decades of PPM research. We identify 17 PPM principles in four categories that have shaped our thinking and show how these are embedded in a semantical hierarchy. By doing so we put light on the thoughts, intentions, and practices behind the principles, which support practitioners in becoming knowledgeable agents. By reflecting on the deficits of the identified principles’ underlying assumptions, we propose a path for future research.