This special issue of U.Porto Journal of Engineering contains selected papers presented at the 3rd International Conference on Materials Design and Applications 2020 (MDA 2020), held online during 5-6 November 2020. The goal of the conference was to provide a unique opportunity to exchange information, present the latest results as well as to discuss issues relevant to materials design and applications. The focus is on fundamental research and application areas in the field of the design and application of engineering materials, predominantly within the context of mechanical engineering applications. This includes a wide range of materials engineering and technology, including metals, e.g., lightweight metallic materials, polymers, composites, and ceramics. Advanced applications include manufacturing of new materials, testing methods, multi-scale experimental and computational aspects. Approximately 155 papers were presented by researchers from nearly 30 countries.