Like many other fields, the area of manufacturing has advanced massively since the onset of the technological revolution brought about by advances in computing and smart technologies, and with the accelerating globalisation of manufacturing in the 21st century, the urgent need to keep pace has produced further rapid advancements in technology, research, and innovation. This book presents the proceedings of ICMR 2023, the 20th International Conference on Manufacturing Research, held from 6 - 8 September 2023 in Aberystwyth, Wales, UK. This annual conference is a friendly and inclusive platform for a broad community of researchers with the common goal of developing and managing the technologies and operations key to manufacturing. As well as bringing together researchers, academics, and industrialists to share their knowledge and experience, the conference also serves to promote manufacturing-engineering education, training and research. Reflecting the context of Industry 4.0 and beyond, the theme of the 2023 conference is sustainability in smart manufacturing environments. More than 68 papers were submitted for the conference, from which the 33 papers presented here were selected and accepted after a rigorous peer review process; an acceptance rate of 49%. The papers are grouped into 8 sections: operations and supply chain management; manufacturing technology; manufacturing and process modeling; robotics and simulation systems; supply chain systems; process characterization and simulation; operations and supply chain management; and design and prototyping. Providing a wide-ranging overview of advances in the field, the book will be of interest to all those working in manufacturing research.