This lecture covers the Kolmogorov K41 theory, including the Navier-Stokes equations, statistics, and three additional hypotheses related to restored symmetries, self-similar scaling, and finite dissipation. The lecture discusses energy transport, the Karman-Howarth-Monin relation, and the Kolmogorov four-fifth law. It also explores the energy spectrum and the energy flux for homogeneous and isotropic turbulence.