This volume presents a current review of knowledge on elementary collision processes of highly excited atoms with neutral particles. It provides the basic physical approaches and theoretical techniques for the description of a broad class of observed collision phenomena in gases and low temperature plasmas involving Rydberg atoms. The author considers different mechanisms of inelastic and quasielastic collisions with a change in the principle and orbital quantum numbers, transitions between the fine-structure components, direct and associative ionisation, ion-pair formation and electron-ion recombination processes.