Product designers can apply different strategies, methods, and tools for sustainable product development. Nature-Inspired Design Strategies (NIDS) offer designers a distinct class of strategies that use ‘nature’ as a guiding source of knowledge and inspiration for addressing sustainability. Biomimicry and Cradle to Cradle, two Nature-Inspired Design Strategies, have already been implemented in product design practice and in curricula of higher education. But how are these strategies applied, and how do they help designers in developing ‘sustainable products’? Based on case-study research of multiple design projects, this study shows how the design philosophy, principles, methods, and tools of Biomimicry and Cradle to Cradle have inspired and guided product designers in their work. The findings demonstrate the value, as well as the current limitations, of Nature-Inspired Design Strategies. The insights and recommendations provided in the thesis are valuable for researchers, designers, and educators with an interest in Nature-Inspired Design, and help to clearly position Nature-Inspired Design in sustainable design practice.