Explore the intricacies of the QCD Axion in this comprehensive lecture by David Marsh from the International Centre for Theoretical Sciences. Delve into topics such as the Strong-CP problem, spontaneous symmetry breaking, and the chiral anomaly. Learn about axion ingredients, the PQ Lagrangian, and axion-fermion coupling. Examine the QCD axion potential, ChPT and axion mass, and lattice QCD applications. Discover various QCD axion models and constraints in this 1 hour 29 minute presentation, part of a program on less-traveled paths of dark matter focusing on axions and primordial black holes.