The established theory of strong interactions, QCD, lacks a cogent
explanation for why it does not violate the charge-parity symmetry to
An elegant mechanism, due to Roberto Peccei and Helen Quinn, comes with
the clear-cut prediction of a new particle, the axion.
Although axions have been searched for since their proposal, in recent
years this field is experiencing a strong increase of interest in the
After reviewing the axion solution of the strong-CP problem and the
experimental status of axion searches, I will focus on some recent
developments in axion model building, showing that the QCD axion
parameter space is much larger than what traditionally thought. The
implications for astrophysical limits and future experiments will be
discussed as well.