  We present a review of the different aspects associated with the interaction
of successive CMEs in the corona and inner heliosphere, focusing on the
initiation of series of CMEs, their interaction in the heliosphere, the
particle acceleration associated with successive CMEs, and the effect of
compound events on Earth's magnetosphere. The two main mechanisms resulting in
the eruption of series of CMEs are sympathetic eruptions and homologous
eruptions. The interaction of successive CMEs has been observed remotely in
coronagraphs and heliospheric imagers, and inferred from in situ measurements.
It is associated with magnetic reconnection, momentum exchange, the propagation
of a fast magnetosonic shock through a magnetic ejecta and changes in CME
expansion. The presence of a CME a few hours before a fast eruption is
connected with higher fluxes of SEPs, while CME-CME interaction occurring in
the corona is often associated with unusual radio bursts. Higher suprathermal
population, enhanced turbulence and wave activity, stronger shocks, and
shock-shock or shock-CME interaction have been proposed as physical mechanisms
to explain these SEP events. When measured in situ, CME-CME interaction may be
associated with relatively well organized multiple-magnetic cloud events,
instances of shocks propagating through a previous magnetic ejecta or more
complex ejecta. The compression of a CME by another and the propagation of a
shock inside a magnetic ejecta can lead to extreme values of the southward
magnetic field, sometimes associated with large values of the dynamic pressure.
This can result in intense geomagnetic storms, but also trigger substorms and
large earthward motions of the magnetopause, potentially associated with
changes in the outer radiation belts. Future measurements by Solar Probe+ and
Solar Orbiter may shed light on the evolution of CMEs as they interact.

Space Physics