This book presents the most comprehensive and methodical work on the theory of the moiré phenomenon, providing a full general-purpose and application-independent exposition of this fascinating effect. Based on the Fourier theory, it leads the reader through the various phenomena which occur in the superposition of repetitive layers, both in the image and in the spectral domains. The first chapters of the book present the basic theory, covering the superposition of monochrome, periodic layers. In later chapters, the theory is extended to the even more interesting cases of polychromatic moirés and moirés between repetitive, non-periodic layers. Throughout the whole text the book favours a pictorial, intuitive approach, which is supported by mathematics, and the discussion is accompanied by a large number of figures and illustrative examples. The prerequisite mathematical background is limited to an elementary familiarity with the Fourier theory.