The linear economy’s “take-make-waste” model causes severe environmental harm, especially in manufacturing – one of the most resource- and energy-intensive sectors. A circular economy offers an alternative, focusing on resource efficiency through reuse, recycling, and design for longevity. As up to 80% of a product’s environmental impact is determined during development, this phase is a critical leverage point for circular implementation. Despite growing recognition of circular design principles, a key barrier is the lack of specific knowledge in product development to shift from linear to circular products. While prior research acknowledges this knowledge gap, its precise nature and impact remain unclear. To address this gap – considering the environmental relevance of manufacturing and the central role of product development as a leverage point – this systematic literature review analyzes existing studies on circular product development to identify and categorize knowledge gaps. Applying predefined selection criteria, the review identified and synthesized seven relevant articles, revealing fundamental deficits. By mapping these gaps into five categories, the study provides a structured foundation for future research and industrial strategies, enabling a more effective transition to circular product development.