  Recent spatially-resolved observations of protoplanetary disks revealed a
plethora of substructures, including concentric rings and gaps, inner cavities,
misalignments, spiral arms, and azimuthal asymmetries. This is the major
breakthrough in studies of protoplanetary disks since Protostars and Planets VI
and is reshaping the field of planet formation. However, while the capability
of imaging substructures in protoplanetary disks has been steadily improving,
the origin of many substructures are still largely debated. The structured
distributions of gas and solids in protoplanetary disks likely reflect the
outcome of physical processes at work, including the formation of planets. Yet,
the diverse properties among the observed protoplanetary disk population, for
example, the number and radial location of rings and gaps in the dust
distribution, suggest that the controlling process may differ between disks
and/or the outcome may be sensitive to stellar or disk properties. In this
review, we (1) summarize the existing observations of protoplanetary disk
substructures collected from the literature; (2) provide a comprehensive
theoretical review of various processes proposed to explain observed
protoplanetary disk substructures; (3) compare current theoretical predictions
with existing observations and highlight future research directions to
distinguish between different origins; and (4) discuss implications of
state-of-the-art protoplanetary disk observations to protoplanetary disk and
planet formation theory.
