We are working on the development of a versatile tool to sense weak magnetic fields by the optical detection of quantum spin coherence and relaxation in nitrogen-vacancy (NV) centers in diamond. This new instrument has far-reaching prospects from condensed matter to biology due to the unique combination of nanoscale spatial resolution, magnetic sensitivity, and variable temperature operation (from mK to well above room-T). In parallel I work on growth of isotopically pure carbon-12 diamond films with nitrogen delta-doping to form NVs a few nanometers from the diamond surface, allowing these single spin sensors to be brought extremely close to external target samples. We have implemented these materials to study noise intrinsic to diamond surfaces and to perform nanoscale magnetic relaxation imaging of electronic spins at room temperature. Future work will focus on improving the magnetometer’s sensitivity, spatial resolution, and measurement protocols while applying it to new sample systems.