Advances in free-floating miniature medical implants promise to offer greater effectiveness, safety, endurance, and robustness than today's prevailing medical implants. Wireless power transfer (WPT) is key to miniaturized implants by eliminating the need for bulky batteries. This paper reviews design strategies for WPT with mm-sized implants focusing on resonant electromagnetic and ultrasonic transmission. While ultrasonic WPT offers shorter wavelengths for sub-mm implants, electromagnetic WPT above 100 MHz offers superior power transfer and conversion efficiency owing to better impedance matching through inhomogeneous tissue. Electromagnetic WPT also allows for fully integrating the entire wireless power receiver system with an on-chip coil. Attaining high power transfer efficiency requires careful design of the integrated coil geometry for high quality factor as well as loop-free power and signal distribution routing to avoid eddy currents. Regulating rectifiers have improved power and voltage conversion efficiency by combining the two RF to DC conversion steps into a single process. Example designs of regulating rectifiers for fully integrated wireless power receivers are presented.