One area in which quantum-based technology is already having a significant impact is in the area of metrology and sensing. Metrology is the science of high-precision measurement. As discussed in the following sections, quantum superposition, entanglement, and squeezing allow unprecedented resolution and sensitivity in a broad range of measurements. These improvements are in part due to the reduction of disruptive effects such as turbulence and dispersion and in part due to the more rapid phase oscillations of entangled states. Applications that are discussed include sensing of gravitational fields, quantum gyroscopes, magnetoencephalography, and atomic clocks.