STIS resides in the +V2 axial bay behind the HST main mirror. Externally, the instrument is refrigerator size, measuring 7 x 3 x 3 ft (2.2 x 0.98 x 0.98 m) and weighs 825 lb (374 kg). Internally, STIS consists of a carbon fiber optical bench, which supports the input corrector optics, focusing and tip/tilt motions, the input slit and filter wheels, the Mode Selection Mechanism (MSM), three detectors; two Multi-Anode Microchannel Plate Array (MAMA) detectors (high-technology photon-counting devices) and one Charge-Coupled Detector (CCD), and sets of mirrors and gratings for each detector.

The optical correction necessary to fix the Hubble main mirror aberration is built into the instrument. As with all HST instruments it possesses a fully redundant electronics chain, commonly referred to as Side 1 and Side 2.

STIS has been designed to work in three different wavelength regions, each with its own detector. Some redundancy is built into the design with overlap in the detector response and backup spectral modes. The MSM, is used to select a wavelength region or mode.

A scientific multi-tasker, STIS can function as a High, Medium or Low resolution 2-D spectroscopy, and can perform limited imaging, as well as a possessing a coronagraphic mode. It contains a variety of optical elements; gratings, filters and a prism.

The resolving power of a spectrograph is the wavelength observed divided by the smallest wavelength difference that can be measured. STIS' wavelength range is 1150 - 10,000 Angstroms or 115 - 1000 nm. It's Field of View (FOV) for the MAMA detectors is 25 x 25, for the CCD is 52 x 52 with all detectors supporting a 1024 x 1024 pixel format.