This up-to-date book provides a lucid and concise introduction to the theory of ordered sets and basic ordered algebraic structures. The treatment is modern, with a slant towards recent developments in the theory of residuated lattices and ordered regular semigroups. Featuring material that has been hitherto available only in research articles, and an account of the range of applications of the theory, there are also many illustrative examples and numerous exercises throughout, making it ideal for use as a course text, or as a basic introduction to the field.