Dr. van Genuchten’s work focuses on the development and application of key theoretical and software tools that describe water flow and contaminant transport in soil and groundwater systems and, in doing so, help to define modern hydrogeology. Software tools include RETC, CXTFIT, STANMOD and especially the HYDRUS codes. The HYDRUS family of codes are now the industry standard for modelling variably-saturated flow and solute transport problems of all types. He also developed the van Genuchten functions, which are the standard for quantitative descriptions of the hydraulic properties of unsaturated porous media, including hydraulic conductivity. In addition, he developed the concept of mobile and immobile fluids to describe flow and transport processes in structured soils and fractured rock. Taken together, this work shows him to be one of the most influential vadose zone hydrologists in the world today.