This monograph is devoted to the theory of hypercomplex systems with locally compact basis. Such systems were introduced by Yu. Berezansky and S. Krein in the 1950s and are a generalisation of the notion of a hypergroup (a family of generalised shift operators) which was introduced in the 1970s. The book gives a state-of-the-art account of hypercomplex systems theory. After the introductory chapter, it treats the Lie theory of hypercomplex systems and examples. Topics covered include Fourier transforms, the Plancherel theorem, the Peter-Weyl theorem, representation theory, duality, Gelfand pairs, Sturm-Liouville operators, and Lie theory. New proofs of results concerning Tannaka-Krein duality and Gelfand pairs are given. On the basis of this theory, new approaches to the construction of harmonic analysis on well-known objects become possible. Audience: This volume will be of interest to researchers and graduate students involved in harmonic analysis and representation theory.