This book constitutes the refereed proceedings of the 9th International Conference on Algorithmic Learning Theory, ALT'98, held in Otzenhausen, Germany, in October 1998. The 26 revised full papers presented were carefully reviewed and selected from a total of 34 submissions. Also included are three invited papers and an introduction by the volume editors. The papers are organized in sections on inductive logic programming and data mining, inductive inference, learning via queries, prediction algorithms, inductive logic programming, learning formal languages, and miscellaneous.