In this paper, we examine the state of the art of the physical foaming of polymers with supercritical fluids with a primary focus on carbon dioxide. We provide a critical analysis of the current research pathways and the main scientific open questions. We discuss the knowledge gaps along with technological challenges for further advances. Perspectives on the foaming of amorphous and semi-crystalline polymers, polymer blends, copolymers, and thermosetting polymers are presented. Challenges pertaining to the improved understanding of the nucleation phenomena and the limitations on the modeling and processing methodologies are discussed.