A discussion on the main challenges designing efficient antennas for bio-implantable communication devices is presented, along with some of the main issues encountered in their characterization. Such devices are used in conjunction with health monitoring or health care systems. Implantable antennas are, by nature, electrically small, and difficulties linked to electrically small antenna design apply. But implants are also located in a lossy host body, which induces a major change of paradigm with classic Electrically Small Antnnas (ESA), as the main design challenge for implantable antennas will be to reach an acceptable efficiency, and not a broad enough bandwidth. In this paper, we present first the main challenges to be met in designing implantable antennas, followed by suggestions for an efficient design procedure. Finally, the specific difficulties in characterizing implantable antennas are emphasized.