  Defects with associated electron and nuclear spins in solid-state materials
have a long history relevant to quantum information science going back to the
first spin echo experiments with silicon dopants in the 1950s. Since the turn
of the century, the field has rapidly spread to a vast array of defects and
host crystals applicable to quantum communication, sensing, and computing. From
simple spin resonance to long-distance remote entanglement, the complexity of
working with spin defects is fast advancing, and requires an in-depth
understanding of their spin, optical, charge, and material properties in this
modern context. This is especially critical for discovering new relevant
systems dedicated to specific quantum applications. In this review, we
therefore expand upon all the key components with an emphasis on the properties
of defects and the host material, on engineering opportunities and other
pathways for improvement. Finally, this review aims to be as defect and
material agnostic as possible, with some emphasis on optical emitters,
providing a broad guideline for the field of solid-state spin defects for
quantum information.
