  All stars are born in molecular clouds, and most in giant molecular clouds
(GMCs), which thus set the star formation activity of galaxies. We first review
their observed properties, including measures of mass surface density, Sigma,
and thus mass, M. We discuss cloud dynamics, concluding most GMCs are
gravitationally bound. Star formation is highly clustered within GMCs, but
overall is very inefficient. We compare properties of star-forming clumps with
those of young stellar clusters (YSCs). The high central densities of YSCs may
result via dynamical evolution of already-formed stars during and after star
cluster formation. We discuss theoretical models of GMC evolution, especially
addressing how turbulence is maintained, and emphasizing the importance of GMC
collisions. We describe how feedback limits total star formation efficiency,
epsilon, in clumps. A turbulent and clumpy medium allows higher epsilon,
permitting formation of bound clusters even when escape speeds are less than
the ionized gas sound speed.
