Miniature wireless implantable bioelectronics provide powerful capabilities for biotelemetry, therapeutics, and neural interfacing. These technologies rely on antennas to communicate with external receivers, yet existing systems suffer from poor radiation performance. We address this issue by studying the through-tissue propagation, deriving the optimal frequency range, and obtaining the maximum achievable far-field radiation efficiency. Three problem formulations are considered with increasing complexity and anatomical realism. Polarization effects of TM and TE modes are investigated using an infinitesimal magnetic dipole and a magnetic current sources, respectively. The optimal operating frequency is found within the [10(8), 3 x 10(9)]-Hz range and can be roughly approximated as f approximate to 2.2 x 10(7)/d for deep implantation (i.e. d greater than or similar to 2 cm). Considering the implantation depth, the operating frequency, the polarization, and the directivity, we show that about an order-of-magnitude efficiency improvement is achievable compared to existing devices.