Order theory is a fascinating subfield of discrete mathematics that connects practical utility and abstract mathematical theory, intuitive considerations, and challenging research problems in sometimes surprising ways. The book provides a motivating introduction to the fundamental concepts and modern trends of the mathematical theory of ordered sets, with the author focusing on particularly interesting topics. Since order theory is both simple and demanding, abstract and applied, intuitive and unimaginable, it is particularly well-suited for students in the second half of their bachelor's degree and at the beginning of their master's studies.