How does confinement affect the catalytic activity of mesoporous materials?

So-called confinement effects in porous materials are known to strongly affect diffusion, phase transformations, catalytic properties, etc. In the field of catalysis, it is generally admitted that antagonistic effects at the mesoscale result in the existence of an optimal catalytic efficiency depending on pore size. Herein we highlight some recent examples of pore size effects and their proposed mechanisms. Confinement studies on periodically organised mesoporous materials must be developed because they should provide a certain clearness of arguments and understanding.