An optical arrangement for oblique plane microscopy, the optical arrangement comprising: an illumination generator arranged to provide a beam of illumination through a first objective lens to illuminate or excite an oblique plane of a sample in use, wherein the first objective lens is also arranged to receive a beam of emitted light from the oblique plane of the sample in use; first and second relay lenses (TL1, TL2) and a second objective lens sequentially arranged to receive the beam of emitted light from the first objective lens and to form, at the focal plane of the second objective lens, an intermediate image having a tilted plane conjugate to that of the oblique plane of the sample; a first mirror located at said focal plane of the second objective lens, arranged to receive and reflect the beam of emitted light; a third relay lens; and an image detector; wherein the second objective lens and the third relay lens are arranged to relay the intermediate image from the first mirror to the image detector; and wherein the optical arrangement further comprises a beam splitter disposed between the second objective lens and the second relay lens, the beam splitter being configured to: direct the beam of emitted light from the second relay lens to the second objective lens, and thence to the first mirror; and direct the reflected beam of emitted light from the second objective lens to the third relay lens and thence to the image detector; the optical arrangement characterised in that the first mirror is also arranged to receive the beam of illumination from the illumination generator and to reflect the beam of illumination through the second objective lens; and in that the beam splitter is further configured to direct the beam of illumination from the second objective lens to the second relay lens, and thence to the first relay lens, the first objective lens and the sample. an illumination generator arranged to provide a beam of illumination through a first objective lens to illuminate or excite an oblique plane of a sample in use, wherein the first objective lens is also arranged to receive a beam of emitted light from the oblique plane of the sample in use; first and second relay lenses (TL1, TL2) and a second objective lens sequentially arranged to receive the beam of emitted light from the first objective lens and to form, at the focal plane of the second objective lens, an intermediate image having a tilted plane conjugate to that of the oblique plane of the sample; a first mirror located at said focal plane of the second objective lens, arranged to receive and reflect the beam of emitted light; a third relay lens; and an image detector; wherein the second objective lens and the third relay lens are arranged to relay the intermediate image from the first mirror to the image detector; and wherein the optical arrangement further comprises a beam splitter disposed between the second objective lens and the second relay lens, the beam splitter being configured to: direct the beam of emitted light from the second relay lens to the second objective lens, and thence to the first mirror; and direct the reflected beam of emitted light from the second objective lens to the third relay lens and thence to the image detector; the optical arrangement characterised in that the first mirror is also arranged to receive the beam of illumination from the illumination generator and to reflect the beam of illumination through the second objective lens; and in that the beam splitter is further configured to direct the beam of illumination from the second objective lens to the second relay lens, and thence to the first relay lens, the first objective lens and the sample. direct the beam of emitted light from the second relay lens to the second objective lens, and thence to the first mirror; and direct the reflected beam of emitted light from the second objective lens to the third relay lens and thence to the image detector; the optical arrangement characterised in that the first mirror is also arranged to receive the beam of illumination from the illumination generator and to reflect the beam of illumination through the second objective lens; and in that the beam splitter is further configured to direct the beam of illumination from the second objective lens to the second relay lens, and thence to the first relay lens, the first objective lens and the sample.