HELIOS will be the world's most sensitive instrument for measuring spectra of inelastically-scattered thermal neutrons. Such data can provide spectra of elementary excitations in materials. It is anticipated that HELIOS will be used for studies of phonons, magnetic excitations, and crystal field excitations. The funding for this instrument is being provided by the U.S. Dept. of Energy through its Office of Basic Energy Sciences. HELIOS be built at the Los Alamos Neutron Science Center (LANSCE), with engineering performed primarily at Argonne, Los Alamos, and Oak Ridge National Laboratories.