Professor McKellar's work on the borderline of nuclear and particle physics has earned him international distinction. He is a pioneer and leader in the study of the part of the nucleon-nucleon force due to the weak interaction. He has made major contributions in the application of meson theory to the study of many-body forces in nuclei, which is necessary for a fully-quantitative understanding of nuclear structure. His recent work on nucleon structure and decay promises to be very significant for particle physics and astrophysics.