Handbook of Hydrogeology is a professional reference book guiding the work and research of hydrogeology and providing enlightenment to hydrogeologists and those working in related fields. This paper introduces the revision and enlargement of Handbook of Hydrogeology (2^nd edition) on the basis of its first edition. It gives a brief account of the background, principles, and necessity of revising the handbook. The content, features and application of the handbook are also addressed.