Four decades after its prediction the axion remains the most compelling solution to the strong CP problem and a well motivated dark matter candidate, inspiring several ultrasensitive experiments based on axion-photon mixing. After reviewing the axion solution of the strong CP problem and the experimental landscape of axion searches, I will focus on some recent developments in axion model building suggesting that the QCD axion parameter space is much larger than what traditionally thought. The implications for astrophysical limits and current experiments will be discussed as well.
