The radiative balance of the troposphere, and hence climate, is influenced strongly by radiative cooling associated with emission of infrared radiation by water vapor, particularly at far-infrared (far-IR) wavelengths greater than 15 micrometers and extending out beyond 50micrometers . Water vapor absorption and emission is principally due to the pure rotation band, which includes both line and continuum absorption. The distribution of water vapor and associated far-IR radiative forcings and feedbacks are well-recognized as major uncertainties in understanding and predicting future climate. Up to half of the outgoing longwave radiation (OLR) from the Earth occurs beyond 15.4 micrometers (650 cm-1_ depending on atmospheric and surface conditions. Cirrus clouds also modulate the outgoing longwave radiation in the far-IR. However, despite this fundamental importance, far-IR emission (spectra of band- integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 micrometers . In this talk we will review the role of the far-IR radiation in climate and will discuss the scientific and technical requirements for far-IR measurements of the Earth's atmosphere.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.