Critical Point Theory for Lagrangian Systems Lagrangian systems constitute a very important and old class in dynamics. Their origin dates back to the end of the eighteenth century with Joseph Louis Lagranges reformulation of classical mechanics. The main feature of Lagrangian dynamics is its variational flavor: orbits are extremal points of an action functional. The development of critical point theory in the twentieth century provided a powerful machinery to investigate existence and multiplicity questions for orbits of Lagrangian systems. This monograph gives a modern account of the application of critical point theory and more spec