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This monograph provides the first survey of Floquet theory for partial differential equations with periodic coefficients. The author investigates, among others, hypoelliptic, parabolic, elliptic and Schri^dinger equations, and boundary value problems arising in applications. In particular, results are given about completeness of the set of Floquet solutions, Floquet expansions of arbitrary solutions, the distribution of Floquet exponents and quasimomentums, the solvability of nonhomogeneous equations, the existence of decreasing or bounded solutions, and the structure of the spectrum of periodic operators. Many of the results discussed here have been available only in research papers until now. The role of the Floquet-Lyapunov theory for ordinary differential equations is well known, but for partial differential equations an analog of this theory has been developed only recently. This theory is of great importance for the quantum theory of solids, the theory of wave guides, scattering theory and other fields of mathematical and theoretical physics. Some chapters devoted to operator theory may be of particular interest to specialists in complex analysis and functional analysis.