The ubiquity and diversity of planets tell us that they can emerge under
an astonishing range of conditions, but until recently, it had not been
feasible to test many of the predictions of planet formation models. While
protoplanets are difficult to image directly, the advent of ALMA has made
it possible to search systematically for disk structures potentially
induced by planet-disk interactions and to use their morphologies to
infer characteristics of the young planet population. Concentric rings
and gaps are the most common type of substructure observed in dust
continuum emission and are found in disks spanning a large range of
ages and stellar host spectral types. The widespread presence of these
structures suggests that giant planet formation can occur readily within
a million years and up to hundreds of astronomical units from the host
star. Meanwhile, spiral structures are detected in only a minority of
disks, and it is still debated whether they are triggered by a perturber,
by gravitational instability, or another mechanism. Nevertheless,
the disparate properties of spiral structures recently observed in
dust and gas suggest that multiple pathways to spiral arm formation
are operating in disks.