Wolfenstein is a particle phenomenologist, a theorist who focuses primarily on connecting theoretical physics to experimental observations. In 1978 he noted that the presence of electrons in Earth and Solar matter could affect neutrino propagation. This work led to an eventual understanding of the MSW effect, which acts to enhance neutrino oscillation in matter. Wolfenstein received the 2005 Bruno Pontecorvo Prize from The Scientific Council of the Joint Institute for Nuclear Research, JINR, for his pioneering work on the MSW effect.