Two guiding options are available, the GMOS On-Instrument Wavefront Sensor (OIWFS) and the telescope's Peripheral Wavefront Sensors (PWFSs). It is strongly recommended that the GMOS OIWFS be used for all but non-sidereal objects. This is because (1) the PWFSs vignette a large part of the GMOS science field and (2) there is significant flexure between GMOS and the PWFSs.