Systems that sense the volume, activity or other parameters of the urinary bladder are described. The systems include at least two electrodes that are implanted at respective locations proximate the wall of the bladder, and may be located substantially opposite each other with respect to a center of the bladder. At least one of the electrodes receives an electrical signal emitted by another of the electrodes. Systems according to the invention detect an impedance through the bladder based on the signal, e.g., based on one or both of the current or voltage of the signal. Based on the detected impedance, the systems sense one or more parameters of the bladder, such as volume, activity, or the like, which influence the impedance. The detected impedance, or bladder parameter information derived from the impedance, may be stored for short- or long-term monitoring, or used to control delivery of a therapy to a patient.