The kind of neutrinos emitted in nuclear beta decay—namely electron antineutrinos (‾ ν e)— are helping scientists implement a diverse range of intriguing applications beyond fun-damental particle-physics research. Like all neutrinos, they're very difficult to detect because they interact so feebly with matter. Nevertheless, they have in recent years begun providing valuable clues about the origin and thermal history of Earth (see PHYSICS TODAY, September 2011, page 14). They are also providing critical information about the fuel cycle in nuclear reactors and, hopefully soon, new insight on heavy-element production in super-novae. Figure 1. Interior of the SNO+ antineutrino detector nearing comple-tion at the Sudbury Neutrino Observatory in Ontario, Canada. The fore-ground acrylic vessel that will hold almost a kiloton of liquid scintillator is surrounded by thousands of phototubes that will look for light flashes signaling antineutrino interactions in the liquid. The detector will be used for particle physics and geology. Two kiloton-sized ‾ ν e detectors are now moni-toring Earth's interior to help geologists determine the abundance and distribution of uranium and thorium—the planet's principal heat-producing radioactive elements. A third big detector will soon join them (see figure 1). They and much smaller detectors can also monitor nuclear reactors far and near. Under consideration is a next generation of even bigger multitask detectors that could con-tribute broadly to astroparticle physics, geology, reactor studies, and fundamental particle physics. All neutrino varieties are impervious to the electromagnetic and strong-nuclear forces, and are at least a million times lighter than the electron (see the box on page 48). But the minuscule interaction cross sections that make them so hard to see allow us to peer deep into the bowels of exploding stars as well as our own planet. Of course, the various neutrino mass and flavor eigenstates, and their metamorphoses, are them-selves subjects of intense study by particle physicists. But in this article, we focus primarily on the useful-ness of neutrinos for geology and nuclear security.