The Mathematical Theory of Information presents a new mathematical theory of information, built on a single powerful postulate: the Law of Diminishing Information. The concept of information is here, for the first time, defined mathematically by adding this postulate to the axioms of the probability theory. The Law of Diminishing Information is founded on a fusion of two fundamental ideas: Carnap and Bar-Hillel's Ideal Receiver' and Shannon'sNoisy Channel'. The Law of Diminishing Information is applied to information technology, game theory, legislation, logic of research, algorithmic information, chaos theory, control engineering, medical tests, and biological evolution. In physics, both the Second Law of Thermodynamics and Schrödinger's wave function are derived from the Law of Diminishing Information. Conventional information theory, that of telecommunications, is analyzed as a special case, and eight conditions for its applicability are listed. The reader will get the essential ideas to understand and use the concept of information. The Mathematical Theory of Information is suitable as a textbook in general information theory for students of technical, scientific, and mathematical subjects. The book is ideal as a supplementary textbook in traditional courses on telecommunications information theory at all levels. The website of the book is www.matheory.info.