The migration of monatomic hydrogen in crystalline silicon is governed by three potential energy surfaces corresponding to the three possible charges states of hydrogen. The H 0 and H + surfaces have minima at the bond-centre site (BC) and give rise to a donor state, whereas the H - surface has minimum at the interstitial tetrahedral site (T) giving rise to an acceptor state. We review the current experimental status regarding these surfaces. The BC-donor and the T-site acceptor are found to exist in two closely related forms, which we explain by a distortion of the energy surfaces when hydrogen is trapped in the silicon lattice at the nearby interstitial oxygen. We show how this oxygen distortion acts as a catalyst for the conversion from H - at T sites to H + at BC sites and retards the migration of H + through BC sites.