  Studies of individual quantum systems, which have led to considerable
progress in our understanding of quantum physics, have traditionally been
associated with atomic gases. In the last decades however, the emphasis has
shifted towards solid-state systems, which are much more practical for
applications. In particular, a new field has recently emerged that is concerned
with the study of quantum systems based on single spins localized near point
defects in crystalline solids. One such system is the nitrogen-vacancy (NV)
defect in diamond. Initially used as an experimental breadboard for testing
concepts of quantum physics and quantum computation, the NV defect was soon
proposed as a sensitive magnetometer, capable of detecting minute magnetic
fields, down to ultimate level of single spins. This atomic-sized magnetometer
can be used as a standalone sensor, or integrated into an imaging system
providing spatial resolution down to the atomic scale. Diamond-based
instruments thus offer new pathways to probe the magnetism of matter from the
mesoscale down to the nanoscale. This book chapter gives an overview of the
field of diamond-based magnetic sensing and imaging, with an emphasis on
already demonstrated applications of this technology. The chapter is divided
into three main sections. In Section 2, the underlying physics and methods of
diamond-based magnetometry are described. Section 3 is devoted to various
experimental implementations that employ this new class of sensors for magnetic
sensing and imaging. Finally, some recent applications are presented in Section
4.
