His research interests were in quantum field theory (the quantum theory of elementary particles) and in the connections between high-energy particle physics and cosmology (the study of the physical universe). He is best known as one of the first to describe the Higgs mechanism (explaining how some but not all elementary particles have mass), and for his research on topological defects (roughly - points, lines or surfaces in space where there is a jump). From the 1950s he was concerned about the nuclear arms race and from 1970 took leading roles in promoting the social responsibility of the scientist.