Abstract: The invention relates to an ionization chamber for ion beams and to a method of monitoring the intensity of an ion therapy beam by way of such an ionization chamber. For that purpose, the ionization chamber includes a chamber housing, a beam inlet window and a beam outlet window, a chamber volume filled with counting gas, a high-voltage anode and a high-voltage cathode, wherein the ionization chamber is constructed flat and sandwich-like from plate-shaped large-surface-area structures of the above-mentioned components, which are aligned orthogonally relative to the axis of the ion beam, and a centrally arranged large-surface-area orthogonally aligned plate-shaped counting anode is surrounded on both sides by a large-surface-area plate-shaped high-voltage cathode consisting of two parallel cathode plates, and the chamber housing consists of a housing frame, which frames a square ionization chamber volume, and on which frame the beam inlet window and the beam outlet window are mounted gas-impermeably.