  We provide an overview of the experimental techniques, measurement
modalities, and diverse applications of the Quantum Diamond Microscope (QDM).
The QDM employs a dense layer of fluorescent nitrogen-vacancy (NV) color
centers near the surface of a transparent diamond chip on which a sample of
interest is placed. NV electronic spins are coherently probed with microwaves
and optically initialized and read out to provide spatially resolved maps of
local magnetic fields. NV fluorescence is measured simultaneously across the
diamond surface, resulting in a wide-field, two-dimensional magnetic field
image with adjustable spatial pixel size set by the parameters of the imaging
system. NV measurement protocols are tailored for imaging of broadband and
narrowband fields, from DC to GHz frequencies. Here we summarize the physical
principles common to diverse implementations of the QDM and review example
applications of the technology in geoscience, biology, and materials science.
