Knowledge of the precise position of implanted devices enables measurements taken by the device to be correctly interpreted and greatly simplifies post-implantation manipulation of the device (e.g. when injecting refills into the reservoir of implanted drug delivery pumps precise knowledge of the position allows the refill to be delivered in one shot, reducing patient discomfort and the probability of infection). When we deliver power to implanted devices a current is induced in the receiving antenna which generates a weak electromagnetic field, the "scattered field". The Implant Positioning System deduces the implant location from spatial variations in this scattered field. This work poses a number of unique challenges in analog and RF circuit design, electromagnetic analysis, and signal processing.