Abstract: An Active Water Phantom is designed to provide fast, accurate, high resolution, complete Quality Assurance of patient-specific treatment plans utilizing intensity-modulated Ion Beam Therapy, prior to their delivery to the patient. The detection medium is a tissue-equivalent water-based liquid scintillator material. A three-dimensional pattern of scintillation light, emitted upon ion beam irradiation, is reconstructed from three orthogonal two-dimensional light yield profiles, which are read out for each individual beam energy layer. The 3-D information has dose measurement accuracy 1-2% and spatial resolution 1-2 millimeters. The measurement sequence provides up to four orders of magnitude more data characterizing the treatment plan than currently commercially available alternatives, all in a time period no greater than that needed for actual delivery of the dose fraction to a patient.