The book intends to give a modern presentation of the classical Markov and Lagrange spectrum, which are fundamental objects from the theory of Diophantine approximations and of their several generalizations related to Dynamical Systems and Differential Geometry. Besides presenting many classical results, the book includes several topics of recent research on the subject, connecting several fields of Mathematics - Number Theory, Dynamical Systems and Fractal Geometry. It includes topics as: Classical results on the Markov and Lagrange spectra: the Markov theorem on the lower spectra The fractal geometry of the complement of the Lagrange spectrum in the Markov spectrum Continuity of Hausdorff dimension of the spectra