Delve into the fundamentals of particle physics, Lagrangians, and Feynman rules in this comprehensive lecture from the ICTP-SAIFR/ExoHad School on Few-Body Physics: Nuclear Physics from QCD. Explore advanced concepts presented by Felipe Ortega-Gama from Lawrence Berkeley Lab, USA, as he delivers the second class in this series. Gain insights into the intricate world of subatomic particles, learn about the mathematical formalism of Lagrangians, and understand the powerful tool of Feynman diagrams for describing particle interactions. This 88-minute talk is designed for physics enthusiasts and researchers looking to deepen their understanding of the connection between quantum chromodynamics (QCD) and nuclear physics.