Summary The ability to determine the three-dimensional location of fluorescently labeled biomolecules in cells with 10 to 70 nm resolution has led to an explosion of discoveries in biology. Super-resolution optical microscopy has led to recent dramatic breakthroughs in our understanding of the organization of molecules in a wide variety of protein assemblies and has led to discoveries of new supramolecular architectures present in organelles. The spatial resolution typically achieved by super-resolution optical microscopy remains, frustratingly, considerably larger than most biomolecules. The goal of this technology development proposal is to create a technology for localizing individual biomolecules with angstrom precision. We propose a technology for localizing molecules using spin labels. The proposed work will employ a magnetic resonance force microscope, in which an attonewton-sensitivity cantilever with a 100 nanometer diameter magnetic tip is operated near a sample surface in high vacuum at cryogenic temperatures. The magnet-tipped cantilever serves two roles. It acts as a force-gradient detector, enabling the observation of magnetic resonance from individual electron spins as a shift of the cantilever's mechanical resonance frequency. It furthermore provides a source of magnetic field gradient, 5 gauss/angstrom or larger, that makes possible the three dimensional magnetic resonance imaging of individual electron spin labels with angstrom spatial resolution. Proof- of-concept data has been acquired demonstrating the ability to detect magnetic resonance from 100's of nitroxide spin labels and to spatially resolve electron spin density at a resolution 100 times smaller than the diameter of the magnetic tip. We present a stepwise technology development plan — backed by theory, simulations, and preliminary data — for achieving the detection of individual nitroxide spin labels and imaging their locations in three dimensions with angstrom precision. Proposed innovations include achieving near-unity spin polarization by operating at high magnetic field and low temperature using novel cryogenic chip-scale microwave sources, employing better inter- ferometric cantilever position detectors and spin modulation schemes to evade sample-related noise, harnessing synchronized cantilever and spin excitation pulse sequences to achieve high fidelity spin modulation, developing robust Bayesian image collection and reconstruction protocols, and fabricating improved cantilevers and magnetic tips for increased per-spin sensitivity. The technology will be validated using well characterized nucleic-acid rulers, biomolecules, protein complexes, and antibodies. Proof-of-concept experiments will be carried out to demonstrate the applicability of the technology to flash frozen biological samples and the ability to carry out correlative fluo- rescent localization experiments. Taken together the proposed work represents a new technology for localizing an individual (spin-l...