Fiber spaces, connections and Yang-Mills fields.- A geometric introduction to Yang-Mills-equations.- Symmetry as a clue to the physics of elementary particles.- Topological excitations in gauge theories, An introduction from the physical point of view.- Particles, fields and quantum theory.- Monopole solitons.- Yang-Mills theory and Gravitation: A comparison.- The twistor description of linear fields.- Twistor techniques in gauge theories.- Simple pseudopotentials for the Kd V-equation.