  The Central Molecular Zone (CMZ) is a ring-like accumulation of molecular gas
in the innermost few hundred parsecs of the Milky Way, generated by the inward
transport of matter driven by the Galactic bar. The CMZ is the most extreme
star-forming environment in the Galaxy. The unique combination of large-scale
dynamics and extreme interstellar medium conditions, characterised by high
densities, temperatures, pressures, turbulent motions, and strong magnetic
fields, make the CMZ an ideal region for testing current star and planet
formation theories. We review the recent observational and theoretical advances
in the field, and combine these to draw a comprehensive, multi-scale and
multi-physics picture of the cycle of matter and energy in the context of star
formation in the closest galactic nucleus.
