Implantable microelectronic systems offer new and innovative approaches for medical diagnostics and therapy. Functional implants for the treatment of cardiovascular diseases acted as pioneer systems and have undergone decades of development. The experience gained has found its way into other medical implants such as orthopedic prostheses or neurophysiological stimulators and has enabled steps toward equipping them with intelligent functions. Recently, highly integrated microelectronic chips have begun to make their way into the human body to act as sensors for metabolites such as glucose or actuators for pain reduction via peripheral nerve stimulation, etc. Challenges that arise concern the biostability of semiconductor surfaces interacting with the body environment. Also to be solved are the tasks of data transmission to the outside world as well as the power supply, both issues being closely related. In addition, challenges recently became obvious in terms of software and data security, since medical implants generate highly sensitive data for the privacy of patients. Here, the technical solutions for this exciting field of bioelectronics as well as perspectives for implantable electronic devices will be presented and societal constraints will be discussed.