Thirty years of solar neutrino observations showed a deficit
relative to models of the Sun's energy production and---even more
suggestively---an energy dependence to that deficit.  While this was
ostensibly a `setback' for solar astronomy, it was a great opportunity
for particle physics, because it raised the possibility that the
apparent deficit was the result of new properties of neutrinos, and that
studies of the Sun could elucidate those properties.  The Sudbury Neutrino
Observatory (SNO) was designed to determine whether the neutrino measurements
were caused by the change of the Sun's electron neutrinos into other
active flavors, by directly comparing exclusive measurements of the flux of
electron neutrinos with the inclusive measurement of the flux of all neutrino
flavors.  SNO's first results, combined with earlier measurements by the
Super-Kamiokande collaboration,  in fact do show a significant difference
between these two measurements, providing direct evidence that neutrinos from
the Sun do change flavor.   SNO is now working on further measurements---such
as the time dependence of the solar flux---which may help to identify neutrino
oscillations as the mechanism for the flavor change.