In this interview, David Zierler, Oral Historian for AIP, interviews John Schwarz, Harold Brown Professor of Theoretical Physics, Emeritus, at Caltech. He describes his family background as a childhood of European emigres, both of whom were scientists, and who escaped Nazi persecution at the beginning of World War II. Schwarz recounts his childhood in Rochester and then on Long Island, and he describes his undergraduate experience at Harvard, where he studied mathematics. Schwarz explains how his interests in the “real world” drew him to physics, which he pursued in graduate school at Berkeley and where he worked with Geoffrey Chew on pursuing a theory of the strong nuclear force. He explains Chew’s conclusion that quantum field theory was not relevant toward developing a theory on the strong nuclear force, and he proposed, alternatively, the S-matrix, which in turn was overtaken by the Yang-Mills gauge theory known as quantum chromodynamics. Schwarz explains how Veneziano’s Eular beta function grew out of the S-matrix program, which extended into a new theory called the dual resonance model, which came to be known as string theory because the model was understood as a kind of quantum theory of one-dimensional objects called strings. Schwarz recounts his contributions to these developments during his time at Princeton, where he collaborated with David Gross, André Neveu, and Joël Scherk. He discusses the significance of Claud Lovelace’s work at CERN, where he found that singularities could be made into poles, and he explains how the second string theory came about in 1971 which required ten spacetime dimensions. Schwarz explains why string theory was not part of the work Glashow and Georgi were doing to unify the three forces of electromagnetism, weak interactions, and strong interactions within a larger gauge symmetry. He describes Feynman’s reluctance in accepting QCD but why, in the end, it proved to be the superior way to explain the strong nuclear force. Schwarz describes his decision to join the faculty of Caltech with the encouragement of Gell-Mann, and he explains the ongoing value of string theory even with QCD firmly established, because it gives gauge theory interactions. He recounts the “second revolution” of string theory in 1984 and his work with Michael Green, and he describes the initial optimism that supersymmetry would be discovered with the advent of the LHC. Schwartz describes Ed Witten’s rising stature in the field, and he shares his views on why thousands of people remain captivated by string theory today. He provides a response to the common criticism that string theory is untestable, and he explains the significance of Juan Maldacena’s discovery of the connection between string theory and conformally invariant field theories. At the end of the interview, Schwarz reviews what among the original questions in string theory he feels have been answered, and which remain subjects of inquiry, including his interest in new approaches to quantum gravity.