  Giant impacts refer to collisions between two objects each of which is
massive enough to be considered at least a planetary embryo. The putative
collision suffered by the proto-Earth that created the Moon is a prime example,
though most Solar System bodies bear signatures of such collisions. Current
planet formation models predict that an epoch of giant impacts may be
inevitable, and observations of debris around other stars are providing
mounting evidence that giant impacts feature in the evolution of many planetary
systems. This chapter reviews giant impacts, focussing on what we can learn
about planet formation by studying debris around other stars. Giant impact
debris evolves through mutual collisions and dynamical interactions with
planets. General aspects of this evolution are outlined, noting the importance
of the collision-point geometry. The detectability of the debris is discussed
using the example of the Moon-forming impact. Such debris could be detectable
around another star up to 10Myr post-impact, but model uncertainties could
reduce detectability to a few 100yr window. Nevertheless the 3% of young stars
with debris at levels expected during terrestrial planet formation provide
valuable constraints on formation models; implications for super-Earth
formation are also discussed. Variability recently observed in some bright
disks promises to illuminate the evolution during the earliest phases when
vapour condensates may be optically thick and acutely affected by the
collision-point geometry. The outer reaches of planetary systems may also
exhibit signatures of giant impacts, such as the clumpy debris structures seen
around some stars.
