Condensed matter is one of the most active areas of research in physics. Ev­ ery year new materials are discovered with properties that are as challenging as or more than those of the year before. The effort is led mainly by exper­ imentalists, who systematically manage to prepare compounds with exotic new properties, including complex ground states involving nontrivial spin, charge, lattice, and orbital arrangements. This work is typically carried out by a relatively small number of researchers, compared with other disciplines, and the interaction between experimentalists and theorists is quite strong. It is a real pleasure for a theorist, like the author, to have experimental data with which to test proposed ideas in short time scales. This allows for a theory-experiment cross-fertilization that keeps the field very active. These are quite interesting times in condensed matter for sure. This book is devoted to the study of a family of materials known as man­ ganites. As for other exotic compounds, the fast development of experiments has induced a rapid evolution of the main theoretical ideas. A considerable ef­ fort, both on theory and experiments, has led to the currently much-accepted notion that nanoscale phase separation is at the heart of the behavior of elec­ trons in these compounds. This point is described in detail in this book, with a plethora of experimental data, computer simulation results, and analytic calculations supporting that description.