After 36 years of solar neutrino experiments, the inescapable conclusion is that a large fraction of the electron neutrinos(&ngr;e)produced by nuclear processes in the Sun's core are metamorphosing into other neutrino varieties somewhere en route to the detectors on Earth (see PHYSICSTODAY, December 2002, page 16). The cause is very likely neutrino oscillation, resulting from the existence of three putative neutrino mass eigen‐states with three different masses. But until recently, the data have allowed considerable latitude in the oscillation parameters, leaving open a variety of possible mechanisms as the neutrinos traverse solar and terrestrial matter as well as 150 million kilometers of vacuum.