A summary of two decades of developments of In Situ remediation is presented in this chapter. The basic principles of In Situ technology application are addressed, such as equilibrium relations between contaminant phases, factors controlling biological and geochemical processes, contaminant characteristics affecting reductive and oxidative conversion parameters and chemical and biological availability. A wide range of In Situ technologies are discussed within the framework of Risk Management. Technologies can be oriented at contaminant sources, migration pathways or at the receptors. Integration of In Situ technologies in sustainable Risk Management approaches is further evaluated relating to the latest concepts and frameworks. Examples are given of application of In Situ technologies in Risk Management approaches, including those for large scale contaminated Megasites. In the future, one can foresee the rise of combined sustainable technologies for soil, groundwater, surface water and energy.