How to store and to search, or retrieve, information efficiently is an important problem in computer science because searching is the most time-consuming part of many programs, and the substitution of a good search method for a bad one often leads to a substantial increase in speed. Particularly, a key search technique of finding which record has a special field as its key has been widely used in database management, compiler construction, natural language processing, and many other applications. The aim of this article is to provide a great deal of references, classified by consideration of their problems.