The Geostationary Lightning Mapper (GLM) under development by Lockheed Martin will be flown as an operational instrument on the Geostationary Operational Environmental Satellite R-Series (GOES-R) spacecraft. GLM is a unique instrument, unlike any other meteorological instrument, both in how it operates and in the information content that it provides. The presentation addresses the unique design of the GLM, the underlying physics that makes lightning observation valuable, and some of the important contributions that that the GLM will make during the GOES-R era.