The time evolution of particles and fields is characterized by a corresponding functional, the action. It is stationary for the real evolution as compared to other imaginable evolutions. This principle of the stationary action allows to understand the conservation of energy, momentum and angular momentum as consequences of symmetries, because to each infinitesimal symmetry of the action there corresponds a conserved quantity and, vice versa, to each conserved quantity there corresponds an infinitesimal symmetry of the action. Physical and geometrical properties, conservation laws and symmetries, are intimately related by Noether’s theorem.