This chapter discusses the principles and concepts involved with on-treatment geometric verification. There are different methods of acquiring an on-treatment verification image, each associated with a different dose and/or energy. The equipment available within a radiotherapy department will have the biggest effect on which on-treatment imaging method can be used. If the treatment volume mainly comprises bony anatomy and gives a large enough view to enable a clear anatomy match, then an megavoltage (MV) image using the treatment field can be used. Kilovoltage imaging delivers a lower radiation dose to the patient than MV large fields. A bony match is using the visible bony anatomy to match the on-treatment image to the reference image. The reference image used will depend on the imaging technique used for treatment. A fiducial marker is a radio-opaque marker that can be implanted into the patient either within or in close proximity to the target volume.