As one of the four cornerstones of its scientific program Horizon 2000, the European Space Agency is studying a far-infrared and submillimeter space telescope (FIRST) that will open to astronomical observations the 100-μm to 1-mm spectral region, virtually unexplored with a subarcminute angular resolution. The 3- to 4-m-diam telescope will be diffraction limited at λ = 300 μm or less. The expected lifetime of this system in orbit is five years or more. The model payload is composed of two focal instruments. The multifrequency heterodyne spectrometer will give a very high spectral resolution in the 400- to 630-μm wavelength range and around 300 μm. The far-infrared receiver (FIR) will be an imaging spectrometer covering the 100- to 400-μm range with spectral resolutions ranging from 3 to 104 obtained with a set of Fabry-Pérot interferometers and filters. Arrays of photoconductors and bolometers are cooled at 1.5 and 0.15 K, respectively, by an open-cycle 3He/4He dilution + Joule-Thomson expansion system developed to operate at zero gravity.