NEUTRINO OSCILLATION HAS BEEN DEMONSTRATED at the Super-Kamiokande lab in Japan to a higher degree of certainty than in previous experiments. Neutrinos, weakly interacting elementary particles only detected for the first time in 1956, are thought by some theorists to reside in a kind of schizoid existence; that is, a neutrino would regularly transform (or oscillate) among several alternative neutrino states, each having a slightly different mass. Such a theory would help to explain the apparent shortfall of neutrinos coming from the Sun. The oscillation proposition has been tested using four neutrino sources: the Sun, Earth's atmosphere, reactors, and particle accelerators. Some tests find tentative but ambiguous evidence for oscillation. Today, at the Neutrino ‘98 conference in Takayama Japan, the Super-Kamiokande collaboration (comprising 100 scientists from 23 institutions in Japan and the US) is announcing the most exacting evidence yet for neutrino oscillation. They study neutrinos made when cosmic rays from outer space strike the upper atmosphere. Some neutrinos, those made overhead above Japan, travel about 20 km or so before entering the underground detector. Other neutrinos, those made in the atmosphere on the far side of the globe, have a travel path of 12,700 km into the detector. In either case, they create, among other things, a high energy electron or muon, which in turn emits a telltale cone of light (Cerenkov radiation) observed by an array of thousands of photodetectors mounted in a tank filled with pure water. Sorting events by electron neutrino or muon neutrino, by high energy or lower energy, and by zenith angle (overhead approach or through the Earth), statistical evidence for oscillation becomes evident. A 1-GeV muon neutrino seems to oscillate every few hundred miles. Four years ago, the same group, using a smaller detector, reported preliminary evidence on the basis of 200 events (Physics Today, Oct 1994). The new report is based on several thousands of events, and provides an approximate mass difference (the test cannot render any neutrino species' mass directly) of about 0.07 eV. Because they are so numerous in the universe, neutrinos, with even a small mass, might play an important role in the formation of galaxies. (Physical Review Letters, 24 August 1998. See http://www.phys.hawaii.edu:80/~jgl/nuosc_story.html, and Physics News Preview, Neutrino Oscillation)