Institute of Theoretical Physics, Department of Physics, Stanford University, Stanford, California
*I first met Professor J. R. Oppenheimer when I went to Berkeley in the fall of 1937 after receiving my Doctor's degree. My next three years of association with him proved to be extraordinarily stimulating. Among other important influences which this period had on my subsequent scientific career was my introduction to general relativity and cosmology. It is a privilege for me to contribute to this issue of theReviews of Modern Physics, and a pleasure to join with others in wishing Professor Oppenheimer a most happy sixtieth birthday.
