  The cosmic ray interaction event generator Sibyll is widely used in extensive
air shower simulations. We describe in detail the properties of Sibyll 2.1 and
the differences with the original version 1.7. The major structural
improvements are the possibility to have multiple soft interactions,
introduction of new parton density functions, and an improved treatment of
diffraction. Sibyll 2.1 gives better agreement with fixed target and collider
data, especially for the inelastic cross sections and multiplicities of
secondary particles. Shortcomings and suggestions for future improvements are
also discussed.
