This special section of the Journal of Geophysical Research is dedicated to Kiyoo Wadati. This paper evaluates his contribution to our knowledge of geophysics and, especially, deep earthquake phenomena. In at least three areas, Wadati wrote several papers before 1936 which profoundly influenced modern geophysics. First, his work provided the first convincing evidence that deep earthquakes existed. Second, he was a leader in the construction of travel time tables and the determination of mantle velocity structure. Finally, he published the first accurate description of the inclined planar zone of deep earthquakes which extends from trenches beneath volcanic island arcs. Wadati's work strongly influenced research of many other scientists both before and after World War II, particularly, Jeffreys, Gutenberg, and Benioff. Several of the questions raised by the research of Wadati and others before 1940 are still unanswered today.