The ESO Very Large Telescope will be equipped with a complementary suite of instruments for infrared imaging and spectroscopy between 1 and 20 micrometers . ISAAC (Infrared Spectrometer and Array Camera) is being developed by ESO and is scheduled to be commissioned during 1997 on the first of the 8 m unit telescopes where it will provide for 1 - 5 micrometers imaging and long slit, low and medium resolution spectroscopy. Its design provides for separate 1 - 2.5 micrometers and 2.5 - 5 micrometers cameras equipped with large format array detectors which can be used either to re-image the telescope focal plane or the intermediate spectrum formed by a grating spectrometer. HgCdTe and InSb arrays of 256 X 256 pixels are currently baselined but the possibility of accommodating larger arrays of up to 1024 X 1024 pixels is foreseen if and when these become available. The complete instrument will be cryogenically cooled using two stage closed cycle coolers and housed in a approximately 1 m diameter vacuum vessel attached to the adapter/rotator at one of the Nasmyth foci. Subject to satisfactory results of prototype tests now in progress it is planned to use diamond turned metal mirrors for the reflecting optics and cryogenic stepper motor drive systems for all moving functions.