The importance of understanding pion nucleus reactions for various fields in nuclear physics is discussed. Specific examples are brought from the areas of relativistic heavy ions, electro- and photo-nuclear reactions and nuclear matter. The status of experimental information about pion nucleus reactions is reviewed. Data on the magnitude of reaction cross sections for the major channels and the systematics of their dependence on various parameters is presented. The question of reaction mechanism is discussed in more details for the inclusive inelastic scattering and true pion absorption. Experiments pertaining to reaction mechanism are reviewed and the question of their sensitivity to the primary interaction process is addressed, Theoretical interpretations of experimental results are presented and different approaches are confronted.