  In this review, I present the case for how massive stars may form through
stellar collisions. This mechanism requires very high stellar densities, up to
4 orders of magnitude higher than are observed in the cores of dense young
clusters. In this model, the required stellar densities arise due to gas
accretion onto stars in the cluster core, including the precursers of the
massive stars. This forces the core to contract until the stellar densities are
sufficiently high for collisions to occur. Gas accretion is also likely to play
a major role in determining the distribution of stellar masses in the cluster
as well as the observed mass segregation. One of the main advantages of this
mechanism is that it explicitly relies on the cluster environment in order to
produce the massive stars. It is thus in a position to explain the relation
between clustered and massive star formation which is not an obvious outcome of
an isolated accretion mechanism. A recent numerical simulation supports this
model as the cluster core increases its density by $10^5$ during gas accretion.
Approximately 15 stellar collisions occur (with $R_{coll}=1$ au) in the cluster
core, making a significant contribution to the mass of the most massive star.
