In this chapter, the influence of mass-transfer and adsorption rates on heterogeneous catalysis is described. As heterogeneous reactions normally occur between molecules adsorbed on the catalytic surface, that is, mainly inside catalyst particles, the concentration of the reactants inside the particles is always lower than that of the fluid bulk. In this way, concentration gradients are originated by the reaction and produce a mass-transfer flux of the reagents and products between the fluid bulk and the particles. The overall reaction rate is, therefore, represented by a sequence of steps: (1) external diffusion, (2) internal diffusion, (3) adsorption, (4) surface chemical reaction, (5) desorption, (6) internal diffusion, and (7) external diffusion. The main heterogeneous mechanisms, like Langmuir–Hinshelwood, are presented and different solved examples and exercises are developed, such as steam reforming of methanol, hydrogenation of iso-octenes, etc. The MATLAB code associated with these examples is available online.