Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Event generators are software libraries that generate simulated high-energy particle physics events. They randomly generate events as those produced in particle accelerators, collider experiments or during the initial phases of the Universe creation. Events come in different types called processes as discussed in the Automatic calculation of particle interaction or decay article. Despite the simple structure of the tree-level perturbative quantum field theory description of the collision and decay processes in an event, the observed high-energy process usually contains significant amount of modifications, like photon and gluon bremsstrahlung or loop diagram corrections, that usually are too complex to be easily evaluated in real calculations directly on the diagrammatic level. Furthermore, the non-perturbative nature of QCD bound states makes it necessary to include information that are outside the perturbative quantum field theory, and well beyond present ability of computation in lattice QCD.