Explore the fascinating world of active matter in this comprehensive lecture that examines materials driven out of equilibrium at the microscopic scale. Delve into how individual units in active systems dissipate environmental energy to inject momentum, creating rich phenomenological behaviors unmatched by passive systems. Learn about standard models of active particles and their collective behaviors through a combination of approximate theories and exact methods. Discover key collective phenomena including motility-induced phase separation and the transition to collective motion, gaining insights into how energetic and mechanical drives create complex dynamics in non-equilibrium systems. Master the theoretical frameworks used to understand these active materials and their unique properties that emerge from microscopic energy dissipation processes.