Assistive and rehabilitative engineering is an interdisciplinary field integrating medicine, therapy, and engineering to promote the independence and social participation of older adults and individuals with disabilities. This chapter provides an overview of the development of assistive devices, considering the International Classification of Functioning, Disability, and Health (ICF). The ICF offers a comprehensive framework for evaluating health conditions and environmental factors. The development process involves user-centered design, focusing on older adults and persons with disabilities, interdisciplinary collaboration, and iterative improvements from prototyping to commercialization. Additionally, this chapter discusses development methodologies for health, medical, and welfare-related devices that incorporate inclusive and barrier-free design concepts, emphasizing living environments that fully accommodate older adults’ and persons with disabilities’ needs.