Biological therapies are transforming the treatment landscape for chronic, autoimmune, and oncological diseases. However, delivering large-volume, high-viscosity biologics subcutaneously remains challenging with conventional approaches. Wearable on-body drug delivery (OBDD) devices address these limitations, enabling patient-administered, home-based delivery of therapeutic volumes exceeding 5–20 mL. This review provides a comprehensive synthesis of OBDD design, product engineering principles, clinical applications, and regulatory considerations. Recent innovations, including adaptive fluidics, electromechanical actuation, and MEMS-based delivery systems, are enhancing precision and usability. Clinical case studies across oncology, immunology, rare diseases, and metabolic disorders demonstrate improved patient outcomes and adherence. Key challenges, such as fluid dynamics, tissue resistance, human factors, regulatory harmonisation, and environmental sustainability, are critically examined. Looking ahead, OBDDs will increasingly integrate digital health technologies, MEMS-based control, and eco-design strategies to support evolving therapeutic needs, including gene and RNA-based therapies. As biological formulations advance, OBDDs are poised to reshape drug delivery paradigms, offering scalable, safe, and patient-centred alternatives to traditional infusion-based care
