THE SPALLATION NEUTRON SOURCE (SNS), costing $1.3 billion, will be built at Oak Ridge National Lab if Congress approves a plan announced recently by Vice President Gore. The SNS would produce neutrons by spallation, a process in which a beam of protons smashes into a target (liquid mercury), releasing a flood of neutrons. Other than the high neutron flux, an important feature of the spallation approach is that the timing of the proton pulses used to make the neutrons permits researchers to determine neutron energies through time-of-flight measurements. Neutrons have no electrical charge and can easily penetrate into crystals for doing structural studies; moreover, neutrons' nonzero magnetic moments make them useful for exploring magnetic fields inside a sample. The SNS will be a sort of neutron microscope: neutron waves with just the right wavelength (a neutron energy of 20 meV corresponds to a wavelength of about 2 angstroms) will illuminate a variety of specimens---new materials, biological objects, superconductors---of great interest to physicists. (Science, 23 January 1998; and Physics World, December 1997.)