Zeolites, known for their crystallinity and porosity, exhibit versatile acid functionalities, making them valuable industrial catalysts. This chapter discusses the acidity of zeolites, including Brønsted and Lewis acids, and their variations. It focuses on the principal methodologies for adjusting zeolite acidity, including isomorphous substitution, steaming, and the impact of extraframework species. The engineering of acid site accessibility and its applications in catalysis, adsorption, and environmental remediation are also discussed. Future perspectives and viewpoints are briefly mentioned.