In 1935, Charles V. Theis published a brief article that was the first transient solution for groundwater flow toward a well. He did so by understanding the vital analogy between groundwater flow and heat transfer. As John Bredehoeft says: “It takes real genius to see the basic form of the underlying theory – this was Theis’ contribution’’. Today, the Theis transient pump test solution is used by all hydrogeologists for well test interpretation. It is one of the many consequences of his discovery. The Theis legacy is not limited to the transient theory. He was one of the first scientists to emphasize the importance of geological heterogeneity. Further information about Theis’ life and discoveries is available in the references provided below.