Spectroscopy is an essential tool in the astronomer's kit, probing the microscopic physics of astronomical objects. The Long-Wavelength Spectrometer (LWS) is one of two complementary spectrometers in the European Space Agency's Infrared Space Observatory (ISO). In this paper, I discuss briefly the scientific considerations which drove the design of the LWS and describe the instrument itself. I then discuss how the instrument works and outline its photometric performance. The measurement of this performance is described in a companion paper at this conference.
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