The Outgoing Longwave Radiation (OLR) emitted to space is a fundamental component of the Earth’s energy budget. There are numerous, entangled physical processes that contribute to OLR and that are responsible for driving, and responding to, climate change. Spectrally-resolved observations can disentangle these processes, but technical limitations have precluded accurate space-based spectral measurements covering the far-infrared (FIR) from 100 to 667 cm−1 (wavelengths between 15 and 100 μm). The Earth’s FIR spectrum is thus essentially unmeasured even though at least half of the OLR arises from this spectral range. The region is strongly influenced by upper tropospheric/lower stratospheric water vapor, temperature lapse rate, ice cloud distribution and microphysics, all critical parameters in the climate system that are highly variable and still poorly observed and understood. To cover this uncharted territory in Earth observations, the Far-infrared Outgoing RadiationUnderstanding and Monitoring (FORUM) mission has recently been selected as ESA’s 9th Earth Explorer mission for launch in 2026. The primary goal of FORUM is to measure, with high absolute accuracy, the FIR component of the spectrally-resolved OLR for the first time with high spectral resolution and radiometric accuracy. The mission will provide a benchmark dataset of global observations which will significantly enhance our understanding of key forcing and feedback processes of the Earth’s atmosphere to enable more stringent evaluation of climate models. This paper describes the motivation for the mission, highlighting the scientific advances that are expected from the new measurements.