  All four giant planets in the Solar system possess irregular satellites,
characterized by large, highly eccentric and/or inclined orbits that are
distinct from the nearly circular, uninclined orbits of the regular satellites.
This difference can be traced directly to different modes of formation. Whereas
the regular satellites grew by accretion within circumplanetary disks the
irregular satellites were captured from initially heliocentric orbits at an
early epoch. Recently, powerful survey observations have greatly increased the
number of known irregular satellites, permitting a fresh look at the group
properties of these objects and motivating a re-examination of the mechanisms
of capture. None of the suggested mechanisms, including gas-drag, pull-down,
and three-body capture, convincingly fit the group characteristics of the
irregular satellites. The sources of the satellites also remain unidentified.
