There is strong evidence that neutrinos have a mass and that oscillations occur among the various neutrino flavors. Several opportunities are ripe for experimental progress. The needed measurements or observations include confirming various effects of neutrino oscillations and identifying the neutrino species involved in each, measuring the values of the mixing parameters responsible for the observed solar neutrino abundances, and measuring the values of the neutrino masses themselves. Answers to these problems are within reach. Much more difficult and subtle issues remain, such as the particle-antiparticle properties of neutrinos and possible CP-symmetry violations in their transitions. New global-scale investigations in the planning stages should culminate in precise results describing these elusive fundamental particles.

Well-founded ideas from unification and particle physics suggest interesting candidates for dark matter, such as neutralinos and axions, with calculable properties. Do these particles exist? Are any of them the actual dark matter observed astronomically? Initial experiments to detect these particles have been mounted, but more sensitivite searches will be needed to detect or rule out these candidates.

Attempts to unify space, time, and matter beyond the Standard Model and general relativity introduce additional interactions and extra space-time dimensions. Tests of the strength of gravity at short range, experiments at particle accelerators, and tests of the principle of equivalence can probe for such signatures of unification.