  In situ measurements of interplanetary coronal mass ejection (ICME)
composition, including elemental abundances and charge states of heavy ions,
open a new avenue to study coronal mass ejections (CMEs) besides remote-sensing
observations. The ratios between different elemental abundances can diagnose
the plasma origin of CMEs (e.g., from the corona or chromosphere/photosphere)
due to the first ionization potential (FIP) effect, which means elements with
different FIP get fractionated between the photosphere and corona. The ratios
between different charge states of a specific element can provide the electron
temperature of CMEs in the corona due to the freeze-in effect, which can be
used to investigate their eruption process. In this review, we first give an
overview of the ICME composition and then demonstrate their applications in
investigating some important subjects related to CMEs, such as the origin of
filament plasma and the eruption process of magnetic flux ropes. Finally, we
point out several important questions that should be addressed further for
better utilizing the ICME composition to study CMEs.
