The GOES-R series is the latest in a long line of American geostationary weather satellites operated by the National Oceanic and Atmospheric Administration (NOAA). The two Geostationary Lightning Mapper (GLM) instruments currently operating on the GOES-16 and GOES-17 satellites give NOAA a unique new capability to map in-cloud and cloud-to-ground lightning flashes across the entire hemisphere within seconds of their occurrence. GLM enables improved warning times for severe weather events, decreased false alarms, persistent coverage over wide geographical areas without sampling bias, and long-term monitoring of trends linked to the changing climate. Viewed from space, emissions from lightning appear as a series of brief (~500 μs) optical pulses diffused through clouds over scales of tens to thousands of km2. A significant portion of the emitted optical radiation is in the form of emission lines, including a prominent neutral atomic oxygen triplet whose emission lines are near 777 nm. GLM discriminates lightning flashes from the bright sunlit cloud background by taking advantage of the spatial, temporal, and spectral characteristics of the optical signature of lightning. This paper describes key design drivers in the development of GLM, methods used to calibrate the instrument, and lessons learned from on-orbit testing. We discuss optimization of the entire signal chain, from the telescope optics to the ground processing algorithms.