The problem that the number of neutrinos detected at the earth as being given off by the sun is far less than the number predicted by theories of nuclear processes in the sun. It is now thought that this is because some of the neutrinos given off by the sun are converted from electron neutrinos to the two other types of neutrinos en route to the earth, in accordance with the expectations of grand unified theories. The correctness of this view has been reinforced by the discovery of transitions between different types of neutrinos in other observations.