Explore the fundamental principles of active matter in this comprehensive lecture that examines materials driven out of equilibrium at the microscopic scale. Learn how individual units dissipate environmental energy to inject momentum into systems, creating rich phenomenological behaviors unmatched in passive systems. Discover standard models of active particles and their collective behaviors through a combination of approximate theories and exact methods. Focus on 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 active systems.