A multireflection time of flight mass spectrometer (MR TOF MS) comprising: a primary ion mirror arrangement including a first primary ion mirror which has electrodes that define a cross section with a first, minor axis of the first primary ion mirror which lies generally in a direction of shift of ions and a second, major transverse axis of the first primary ion mirror, each axis orthogonal to a longitudinal axis of the first primary ion mirror, wherein the first primary ion mirror extends a greater distance in its major axis than in its minor axis and the longitudinal axis being defined generally in the direction of TOF spread of ions in the first primary ion mirror, the first primary ion mirror having a first plane of symmetry that contains the longitudinal axis of the first primary ion mirror and the major axis of the first primary ion mirror; a secondary ion mirror arrangement including a first secondary ion mirror which has electrodes defining a cross section with a first, minor axis of the first secondary ion mirror and a second, major axis of the first secondary ion mirror, each orthogonal to a longitudinal axis of the first secondary ion mirror again defined generally in the direction of TOF separation of ions in the first secondary ion mirror; means for supplying a voltage to the electrodes of the primary and secondary ion mirror arrangements (10, 20) so as to establish electric fields therein such that the electric field in the first primary ion mirror is configured to cause: (a) ions to drift in a direction parallel to the minor axis of the first primary ion mirror; (b) the mean trajectory of ions to intercept the first plane of symmetry of the first primary ion mirror; (c) spatial focussing of the ions in a direction parallel to the direction of shift of ions in the mirror; an ion acceleration region; and an ion guiding means (30, 150) configured to introduce ions from the ion acceleration region into the MR TOF MS in a direction which is non-parallel to the first plane of symmetry of the first primary ion mirror so as to cause ions so introduced to reflect between the primary and secondary ion mirror arrangements at least once prior to exiting them for subsequent processing or detection characterised in that the major axis of the first primary ion mirror is orthogonal to the major axis of the first secondary ion mirror. a primary ion mirror arrangement including a first primary ion mirror which has electrodes that define a cross section with a first, minor axis of the first primary ion mirror which lies generally in a direction of shift of ions and a second, major transverse axis of the first primary ion mirror, each axis orthogonal to a longitudinal axis of the first primary ion mirror, wherein the first primary ion mirror extends a greater distance in its major axis than in its minor axis and the longitudinal axis being defined generally in the direction of TOF spread of ions in the first primary ion mirror, the first primary ion mirror having a first plane of symmetry that contains the longitudinal axis of the first primary ion mirror and the major axis of the first primary ion mirror; a secondary ion mirror arrangement including a first secondary ion mirror which has electrodes defining a cross section with a first, minor axis of the first secondary ion mirror and a second, major axis of the first secondary ion mirror, each orthogonal to a longitudinal axis of the first secondary ion mirror again defined generally in the direction of TOF separation of ions in the first secondary ion mirror; means for supplying a voltage to the electrodes of the primary and secondary ion mirror arrangements (10, 20) so as to establish electric fields therein such that the electric field in the first primary ion mirror is configured to cause: (a) ions to drift in a direction parallel to the minor axis of the first primary ion mirror; (b) the mean trajectory of ions to intercept the first plane of symmetry of the first primary ion mirror; (c) spatial focussing of the ions in a direction parallel to the direction of shift of ions in the mirror; (a) ions to drift in a direction parallel to the minor axis of the first primary ion mirror; (b) the mean trajectory of ions to intercept the first plane of symmetry of the first primary ion mirror; (c) spatial focussing of the ions in a direction parallel to the direction of shift of ions in the mirror; an ion acceleration region; and an ion guiding means (30, 150) configured to introduce ions from the ion acceleration region into the MR TOF MS in a direction which is non-parallel to the first plane of symmetry of the first primary ion mirror so as to cause ions so introduced to reflect between the primary and secondary ion mirror arrangements at least once prior to exiting them for subsequent processing or detection