The molecular cloud lifecycle
  Giant molecular clouds (GMCs) and their stellar offspring are the building
blocks of galaxies. The physical characteristics of GMCs and their evolution
are tightly connected to galaxy evolution. The macroscopic properties of the
interstellar medium propagate into the properties of GMCs condensing out of it,
with correlations between e.g. the galactic and GMC scale gas pressures,
surface densities and volume densities. That way, the galactic environment sets
the initial conditions for star formation within GMCs. After the onset of
massive star formation, stellar feedback from e.g. photoionisation, stellar
winds, and supernovae eventually contributes to dispersing the parent cloud,
depositing energy, momentum and metals into the surrounding medium, thereby
changing the properties of galaxies. This cycling of matter between gas and
stars, governed by star formation and feedback, is therefore a major driver of
galaxy evolution. Much of the recent debate has focused on the durations of the
various evolutionary phases that constitute this cycle in galaxies, and what
these can teach us about the physical mechanisms driving the cycle. We review
results from observational, theoretical, and numerical work to build a
dynamical picture of the evolutionary lifecycle of GMC evolution, star
formation, and feedback in galaxies.
