The sustainable conversion of biomass into biofuels, chemicals and biomaterials has become increasingly important to ensure the well-being of present and future generations. Among the various technologies currently available for biomass recovery, microwave-assisted hydrothermal conversion has recently emerged as a state-of-the-art technology capable of producing a wide range of reaction products for the energy, pharmaceuticals – and the chemical sector. This emerging technology combines the inherent benefits of microwave heating with the sustainable features of hydrothermal utilization of biomass. In this critical overview, for the first time, all work carried out to date on the use of microwave-assisted hydrothermal processes (including microwave-assisted carbonization, liquefaction and treatment / hydrolysis) to convert biomass into hydrochar, bio, is summarized and analyzed crude oil (bio-oil) and valuable chemicals. In this work, important information was compiled in particular, which deals with the influences of the reaction conditions (temperature, time, amount and type of catalyst, loading and type of biomass as well as microwave power) on the yields and key properties of the reaction products. The relationships between these processing parameters and the chemical conversions involved in the processes (hydrolysis, dehydration, decarboxylation, condensation and repolymerization) have been extensively described, and reliable comparisons have also been made between microwave-assisted and conventional hydrothermal technologies at data we are available. As a result, this critical review gathers vital information about the use of this cutting-edge, recently released microwave-assisted hydrothermal technology and paves the way for its expansion and future development and commercialization.