Because of the large real permittivity and significant conductivity of biological tissues at microwave frequencies, the performance of microwave antennas loaded with the tissues is very different from that in air. Moreover, for performing heating and radiometry inside the tissues, the specific absorption rate (SAR) profile generated in the near-field of the antennas is the quantity of interest rather than the radiation pattern. On the other hand, for medical implants, the SAR profile and the tissue loss are undesired effects, but are unavoidable and greatly impact the radiation property and the gain of the antennas. This paper highlights some of these mentioned issues. It presents planar antennas utilized for an intracranial pressure monitoring (ICP) implant and radiometer/heating antennas applied for the measurement of blood perfusion. Measurement setups and/or procedures for the characterization of the antennas will be discussed as well.