Catalysis is the acceleration of a chemical reaction by a small quantity of a substance, which may take part in the reaction, but in the end is not changed by the reaction. This alien substance can be dissolved in the reaction mixture or one of its components, which is referred to as homogeneous catalysis. It can also form its own separate phase, yielding heterogeneous catalysis. Homogeneous and heterogeneous catalysis are equally important from an industrial point of view. Homogeneous catalysis will not be the subject of this book. We will focus on heterogeneous catalysis, where the catalyst is a porous material itself or is impregnated in such material.