This review covers the fundamentals of zeolite materials science and their application as catalysts. After a brief introduction into their structures, the most important parameters are discussed which allow the preparation of an almost infinite variety of zeolitic materials tailored for a given catalytic application. Zeolites are solid acids, and the chemical nature, the density, strength and location of the acid sites in zeolites are discussed. Shape-selective catalysis, which is a unique feature of zeolites, is briefly addressed.