Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Particle swarm optimization (PSO) is a method for doing numerical optimization without explicit knowledge of the gradient of the problem to be optimized. PSO is due to Kennedy, Eberhart and Shi and was originally intended for simulating social behaviour, but the algorithm was simplified and it was realized that the particles were actually performing optimization. The book by Kennedy and Eberhart describes many philosophical aspects of PSO and swarm intelligence. PSO optimizes a problem by maintaining a population of candidate solutions called particles and moving these particles around in the search-space according to simple formulae. The movements of the particles are guided by the best found positions in the search-space, which are continually being updated as better positions are found by the particles.