This book offers a timely report on an emerging topic in the field of wearable assistive technology: the design and development of robotic extra fingers. After a concise review of the state of the art and a description of earlier prototypes, it discusses the authors' efforts to address issues such as portability and wearability of the devices, including strategies to reduce fatigue and to integrate the motion of the extra fingers with that of the human hand. The book also explores optimized control algorithms and the design of wearable sensorimotor interfaces, and presents a set of tests carried out on healthy subjects and chronic stroke patients. Merging concepts from robotics, biomechanics, human factors and control theory and offering an overview of supernumerary robotic fingers, including the challenges, this book will inspire researchers involved in the development of wearable robotic devices and interfaces based on the principles of wearability, safety, ergonomics and user comfort.