M THEORY considers that all the matter in the universe consists of combinations of tiny membranes which come in various dimensionalities. M theory has largely subsumed an earlier theory (circa 1984/85) in which elementary particles were thought of as minuscule vibrating strings (1D membranes, or "1-branes"). Because string theory was able to tame the "infinities" inherent in calculations involving the gravitational interaction, it inspired optimism that the two great pillars of 20th century physics, quantum mechanics and general relativity, could at last be united. Then mathematical difficulties intruded and enthusiasm cooled. But now in what Edward Witten (Institute for Advanced Study) calls the "second string revolution," M theory has shown that the various alternative string models are actually equivalent forms of the same underlying theory. Furthermore, progress has been made in assimilating that most severe of all gravitational systems, the black hole, into a quantum framework. For example, by treating a black hole as a combination of branes, physicists have shown that quantum calculations of a black hole's entropy are equivalent to the entropy calculated using a general relativity approach. Another test of quantum/gravity compatibility is the question of information. Quantum mechanics says that, in the absence of a physical measurement, information in a system cannot be lost. But when matter (an information-rich encyclopedia, say) is swallowed by a black hole, energy is returned to the rest of the universe, if at all, in the degraded form of "Hawking radiation," particles produced pairwise at the very edge of the black hole. Some have argued that even in this case information is not totally lost since it might be imprinted into the Hawking radiation in some way. Witten believes that out of this struggle with gravity a new theory, just as original and powerful as quantum mechanics, might emerge. (Background: Physics Today, April 1996, March and May 1997; a summary of a recent meeting can be found in Science, 27 June.)