This lecture provides a comprehensive overview of the theory behind hadronic contributions to the muon g-2, covering topics such as the Hadronic Vacuum Polarization and Hadronic Light-by-Light contributions. The instructor discusses the historical measurements, present status, and the impact of these contributions on the experimental vs. Standard Model results. Various methods, including data-driven and lattice approaches, are explored to calculate these contributions, highlighting the challenges and uncertainties associated with hadronic physics.