Since the introduction of implantable pacemakers in the early 1960s, implantable medical devices have become more and more interesting for healthcare services. Nowadays, the devices designed to monitor physiological data from inside the human body have great promises to provide major contributions to disease prevention, diagnosis and therapy. Furthermore, minimally invasive devices allow reducing hospitalization terms, thus improving the patients' quality of life. Planning how transmitting information from inside the body to the external world requires a multidisciplinary approach. Such a challenging task combines concepts, models and applied solutions drawing from several fields, including electromagnetism, electronics, biology, and package engineering. More specifically, this work focuses on antennas to be integrated in implantable devices with far field data telemetry capabilities. Thus, in collaboration with the Laboratory of Microengineering for Manufacturing 2 and the Laboratory of Stem Cell Dynamics at the École Polytechnique Fédérale de Lausanne, we have designed, assembled and successfully tested in vivo a Body Sensor Node for telemedicine use. Among its components, the antenna plays a key role. The presence of the human body, which is "hostile" to radio frequency propagation, the need of miniaturization and the necessity of biocompatibility participate all in determining the final characteristics of implantable antennas. Taking into account a wide range of technical and medical concerns, this thesis addresses the analysis, design, realization, in vitro characterization and in vivo testing of such radiators. The analysis is built upon the fundamental theory of antennas in lossy matter, the features of electrically small radiators and the modeling of the human body. This approach allows a clear identification of the main challenges related to implantable antennas, thus setting a solid base for the work presented further on. For instance, biocompatible insulation was found of paramount importance. Accordingly, we have elaborated and implemented physical and mathematical models showing that the proper choice of insulating layers substantially improves the radiation efficiency. The design of implantable antennas takes into account theoretical inputs, packaging considerations and technological constraints. Thus, we propose an effective design strategy that combines these three aspects, and that has been applied to design the Multilayered Spiral Antenna. This radiator integrates with the necessary electronics, power supply and bio-sensor so as to form a Body Sensor Node. In vitro characterization is discussed and carried out for the implantable antenna itself, as well as for the entire implantable device. More specifically, the former characterization is detailed so as to give the possibility of prototyping antennas at the component level. Finally, in vivo testing of the proposed Body Sensor Node was performed in a porcine animal. We believe that our results pave the way for future research oriented to the making of complete telemedicine systems.