A 2.4 meter long, doubly curved, grazing incidence mirror is being developed for a 30 meter high resolution small angle neutron scattering (SANS) instrument now in operation at NIST's Cold Neutron Research Facility. A focussing mirror can in principle yield a higher flux at the sample for a given angular resolution than the pinhole collimation normally used in SANS instruments, provided that a sufficiently sharp focus can be achieved. The mirror is being constructed from 8, 30 cm long, concave cylindrical quartz segments that have surfaces with a mean roughness, (alpha) s, ranging from 0.3 to 0.9 nm. The mirror segments will be aligned along an arc and bent slightly to approximate an overall toroidal shape. The results of design calculations for the mirror system; of neutron reflectivity measurements of the specular and nonspecular scattering from the individual mirror segments and from test pieces with nickel coatings; and tests of the mirror bender are described. From these results, the overall gain in flux that can be realistically expected from the completed mirror system is estimated.