A multi-reflection mass spectrometer is provided comprising two ion-optical mirrors, each mirror elongated generally along a drift direction (Y), each mirror opposing the other in an X direction, the X direction being orthogonal to Y, characterized in that the mirrors are not a constant distance from each other in the X direction along at least a portion of their lengths in the drift direction. In use, ions are reflected from one opposing mirror to the other a plurality of times while drifting along the drift direction so as to follow a generally zigzag path within the mass spectrometer. The motion of ions along the drift direction is opposed by an electric field resulting from the non-constant distance of the mirrors from each other along at least a portion of their lengths in the drift direction that causes the ions to reverse their direction.