Reactions of solids are in general rather complex and involve series-parallel of reactive and physico-chemical processes. The complexity of these reactions can be approached, both experimentally and modeling with different levels of detail. It is essential to recognize, within this complexity, what are the phenomena which really impact on the performance and design of a chemical reactor or process. Different case studies will be presented: iron for H2 production, biomass conversion, tobacco smoking devices, polymers for rockets propellants etc.