Implantable Medical Devices (IMDs) could fulfill many different functions in the human body. Batteries have always been the main power source of these devices. Batteries have some drawbacks, the biggest one being the replacement of IMDs with fully discharged batteries. In this thesis a wireless power transfer system, with a receiving power conversion system with a maximum volume of 10 mm3, is proposed for IMDs implanted at more than 10 cm deep. Ultrasonic wireless power transfer to IMDs could enable scaling down the devices to dimensions less than 1 cm. Compared with the electromagnetic far-field and near-field wireless power transfer methods, the power throughput to the IMD is much higher. Piezo-electric elements could be used to perform the acoustic to electric energy conversion. With the Butterworth-Van Dyke modeling technique, the ultrasonic wireless power transfer link could be characterized. Maximum acoustic intensity at the receiving piezo-electric element is assumed because of the high efficiency and the possible methods to focus the waves from the transmitter at a certain point in the human body. An electrical storage element is assumed because of fluctuations both in received and used power. As the piezo-electric receiver outputs sinusoidal voltage and current waveforms, and the electrical storage element requires DC voltage, the signal requires significant processing for maximum power transfer. A perfect complex conjugate match between the piezo-electric receiver and the power conversion circuit is required for maximum power transfer as well. Two different methods which are used in prior art to achieve maximum power transfer are presented in this thesis, and one new method is developed. The standard method, which is used often in literature, does not include any special processing by means of impedance transformations and is therefore only efficient around one power level operating point. The varying frequency method varies the frequency so as to vary the piezo resistance and match it to the power conversion systems resistance. The piezo inductance varies with the varying piezo resistance, and is canceled out by tunable capacitor banks. A DC boost converter is required for high efficiency to the electric storage element. This method has many drawbacks, among which the continuous communication back to the transmitter to close the loop, which is consuming power and adding delay. The newly proposed method is the AC boost method. The AC voltage of the piezo-electric receiver is transformed into a pulse width modulated square wave voltage, so all power could go through the rectifier and into the storage element. A continuous resistive match is made between the AC boost converter and the piezo-electric element, ensuring maximum power transfer. This new method is validated by performing electric circuit simulations. Circuits are designed for the simulations that are comparable because of using the same piezo-electric element, rectifier, and storage element. The simulations show that it is the most efficient method for a wide load power range of 10 µW to 5 mW. The standard method is as efficient but only at a small range at high power level. The varying frequency method is much more complex and has, therefore, lower power efficiency at high power levels whereas at low power levels it has the same power efficiency.