In the development of implantable bioelectronics, the establishment of efficient wireless RF links between implants and external nodes is crucial, providing substantial contributions to the advancement of medical diagnosis, therapies, and basic science. Implantable antennas, however, encounter formidable challenges due to the coupling of electromagnetic fields with lossy biological tissues, resulting in significantly decreased radiation efficiency. In response to these challenges, this thesis undertakes an exhaustive exploration of implantable antennas, with particular emphasis on the theoretical analysis of loss mechanisms and radiation performance. Considering a broad spectrum of application concerns, we elucidate the constraints imposed by the host body and propose innovative design benchmarks and strategies aimed at achieving efficiency in implantable wireless systems. The conducted studies offer a thorough theoretical analysis of implantable antennas using analytical modeling. In this approach, the implantable antenna is modeled as an elementary source, allowing for an examination of the complex impacts of surrounding biological tissue on the electromagnetic field generated by the source. Utilizing analytical techniques in electromagnetics, including the spherical wave expansion method and Green's functions for multi-layered planar media, we investigate body models with spherical and planar geometries, respectively. This analytical approach yields efficient and accurate analytical representations of electromagnetic fields in both lossy media and free space, providing physical insights for implantable antenna design. Within this thesis, the primary challenges of implantable antennas are addressed through theoretical analysis, thereby proposing design benchmarks and practical strategies for the applications in implantable bioelectronics. The first contribution is the developed analytical approximation for a quick assessment of in-body path loss in the shortest wireless link from the implant to the body-air interface, addressing more specifically the near-field losses for both deep and shallow implantation scenarios. An understanding of the inevitable losses caused by lossy media is valuable in the initial design phase of implantable antennas to achieve optimal radiation, including determining the optimal frequency and source type. The analytical approximation to path loss is further extended to free space by accounting for electromagnetic refraction at the body-air interface, allowing for a quick assessment of wireless transmission efficiency between the implant and the external node. Concerning far-field radiation performance, investigations into radiation patterns of implants within large and small-scale host bodies offer crucial insights for optimizing transmission efficiency in implantable wireless systems. Simple closed-form approximate expressions are derived that enable fast calculation of radiation patterns and maximum gain for antennas implanted in large host bodies. By observing the radiation patterns of implants in rodents, we found that the small host body behaves as a dielectric resonator, leading to unexpected deformations in the radiation patterns. Furthermore, in a specific neural implant development, the co-design of an implantable antenna and power amplifier showcases a miniature implantable wireless device, facilitating 500-Mbps high-data-rate neural recording with a meter-level wireless transmission range.