UIST is a facility class near-infrared imager and spectrometer, built at the UK Astronomy Technology Centre (UKATC) in Edinburgh, and now in use at the UK Infrared Telescope (UKIRT). UIST operates at wavelengths of 1-5 μm, providing a variety of imaging and spectrometry modes. UIST is the first instrument to include a cryogenic deployable integral field unit (IFU), allowing integral field spectroscopy to be carried out over a 3.3 x 6.0 arcsec field of view using any of the grisms available for spectroscopy in UIST. The optical components of the image slicing IFU were tested and aligned on the bench before the IFU was integrated into UIST for cryogenic tests in the laboratory in Edinburgh and on the telescope. These tests included measurements of the image quality produced by the IFU and the transmission of the IFU relative to a slit of equivalent width as a function of wavelength, found to increase from 0.4 at 1 μm to 0.62 at 2.5 μm. When the seeing is poor and high spectral resolution is required the loss of light in the IFU may be significantly less than the slit-losses from a conventional slit. The conditions under which use of the IFU may be preferable to use of a silt are discussed. The data reduction methods used to automatically combine IFU observations with arc-lamp spectra, flat-field frames and standard-star spectra in order to transform the two-dimensional output of the IFU into a calibrated (x, y, l) datacube in near real-time and the procedures required to obtain the necessary calibration data are outlined. An example of one type of observation made possible by the IFU is shown by observations of H₂ lines excited in bow-shocks in the outflow from a young star in the vicinity of the ultra-compact H II region G25.65-1.05, allowing measurement of the spatial variation of the excitation temperature in these shocks.