Hydrated magnesium silicate materials are promising binding materials for applications as alternative cementitious materials and may offer a lower carbon footprint than conventional binders if carbon-free feedstocks are used. The reaction of reactive MgO with silica sources yields magnesium silicate hydrate (M-S-H) phases. These M-S-H phases have layered silicate structure and are stable from pH 8 to 12; thus, M-S-H phases are of interest for applications requiring low-pH binders, such as radioactive waste management and encapsulation. Reactive Mg-cements have been extensively studied in the past 100 years, yet the literature does not offer a dedicated review of M-S-H materials. This review provides a critical analysis of the current knowledge on M-S-H-based binders, discussing the main variables involved in M-S-H synthesis, the structure and characterization of these phases, thermodynamic parameters, stability, as well as the mechanical properties and potential applications of the resulting binders.