  The presence of carbon-chain molecules in the interstellar medium (ISM) has
been known since the early 1970s and $>130$ such species have been identified
to date, making up $\sim 43$% of the total of detected ISM molecules. They are
prevalent not only in star-forming regions in our Galaxy but also in other
galaxies. These molecules provide important information on physical conditions,
gas dynamics, and evolutionary stages of star-forming regions. Larger species
of polycyclic aromatic hydrocarbons (PAHs) and fullerenes (C$_{60}$ and
C$_{70}$), which may be related to the formation of the carbon-chain molecules,
have been detected in circumstellar envelopes around carbon-rich Asymptotic
Giant Branch (AGB) stars and planetary nebulae, while PAHs are also known to be
a widespread component of the ISM in most galaxies. Recently, two line survey
projects toward Taurus Molecular Cloud-1 with large single-dish telescopes have
detected many new carbon-chain species, including molecules containing benzene
rings. These new findings raise fresh questions about carbon-bearing species in
the Universe. This article reviews various aspects of carbon-chain molecules,
including observational studies, chemical simulations, quantum calculations,
and laboratory experiments, and discusses open questions and how future
facilities may answer them.
