Two and a half decades ago, the discovery of Mercury’s polar deposits from Earth-based radar observations provided the first tantalizing, but limited, evidence for the possibility of water ice on the solar system’s innermost planet. Identifying the materials in Mercury’s polar deposits was one of the six major science questions that originally motivated the MESSENGER mission. In the course of the mission’s more than four Earth years of operations in orbit about Mercury, MESSENGER produced multiple datasets to investigate Mercury’s polar deposits: determinations of regions of permanent shadow, neutron spectrometer observations, laser altimeter reflectance measurements, thermal model results, and direct images of the deposits. These datasets provided compelling evidence that in addition to substantial amounts of water ice stored in Mercury’s polar deposits, there are also other volatile materials, postulated to be dark, organic-rich compounds that bury the water ice deposits. This chapter reviews MESSENGER’s investigations of Mercury’s polar deposits and discusses the resulting implications for the origin and evolution of Mercury’s polar water ice.