Semiconductor-based Implantable Prosthetic Devices are artificial devices embedded into the body to replace/rectify a functionality of a part of the body. The devices function as prosthetic devices by the operation of electronics, which is their core. This chapter describes such devices with specific emphasis to the design and functioning of the electronics. The chapter begins with an overview of existing implantable devices, both commercialized and those under development. The components of a generic implantable prosthetic device are described leading to a detailed discussion of the major components in the continuing sections. Design examples will be presented from an implantable retinal prosthetic device under development for replacing lost vision. The concluding section of the chapter presents a discussion on the challenges and future direction of these implantable devices.