Abstract: In recent years, optically probed nitrogen–vacancy (NV) quantum defects in diamond have become a leading modality for magnetic, electrical, and temperature sensing at short length scales (nanometers to millimeters) under ambient conditions. This technology has wide-ranging application across the physical and life sciences — from NMR spectroscopy at the scale of individual cells to improved biomedical diagnostics to the search for dark matter. I will provide an overview of quantum diamond sensors and their diverse applications.