Implanted medical devices (IMDs), in particular neuro-stimulators, drug delivery chips and cochlear implants are undergoing miniaturization. Some of these miniaturized IMDs are “active” in the sense that they require a power source for operation. In most cases, the ideal power source needs to be an implanted battery of dimensions similar to that of the device. The state-of-the-art of battery miniaturization is reviewed with emphasis on novel Li and Li-ion two- and three-dimensional thin-film microbatteries. It is shown that three-dimensional thin-film batteries may provide a solution to the power requirements of miniaturized IMDs.