The present paper presents the phenomena occurring in fixed bed reactors spanning, from the small scale of single pellet, where reaction and diffusion are competing, to the macroscale of whole apparatus, where dispersion and heat transfer play an important role; the most important models used in describing the behaviour of fixed bed reactors and the dependence of most relevant parameters from the geometrical characteristics of the reactor and from the physical properties of the reacting gases are examined. Advice is given in order to design the reactor choosing the governing parameters in order to have stable and effective performance, avoiding situations with multiple solutions and hence potential instability problems.