Paul Witherspoon began his career as a petroleum engineer where he is best known for his work on the assessment of caprock integrity for underground storage of natural gas. It was only after joining the faculty of the University of California at Berkeley at the age of 38 that he began to address hydrogeological issues. He was the leader in turning hydrogeological attention away from a singular concern with aquifer assessment toward a greater consideration of the importance of aquitards. The aquitard-evaluation methodologies he and his students developed at Berkeley, and later at the Lawrence Berkeley Laboratory, turned out to be invaluable in the assessment of contaminant transport problems and nuclear waste isolation. He and his LBL team are also widely recognized for their pioneering work on flow in fractured rocks, geothermal energy resources, and numerical simulation methods. Paul has always been a strong advocate of large-scale underground testing, and the importance of the in-situ research carried out at the Stripa Mine in Sweden under his influence attests to the validity of his vision.