Young tableaux are fillings of the boxes of diagrams that correspond to partitions with positive integers, that are strictly increasing down columns and weakly increasing along rows. The aim of this book is to develop the combinatorics of Young tableaux and to show them in action in the algebra of symmetric functions, the representations of the symmetric and general linear groups, and the geometry of flag varieties. Many of these applications have not been available in book form. In the first part of the book the author develops the basic combinatorics of Young tableaux, including the remarkable constructions of "bumping" and "sliding" that can be used to make them into a monoid, and several interesting correspondences. In Part II these results are used to study representations of the symmetric and general linear groups. In Part III we see relations with geometry on Grassmanians and flag manifolds, including their Schubert subvarieties, and the related Schubert polynomials. Two appendices contain variations of the combinatorics of Part I and the topology needed to relate subvarieties to cohomology classes. The combinatorial chapters of the book are self-contained so that students will find the discussion easy to follow. Researchers in representation theory and algebraic geometry as well as in combinatorics will find Young Tableaux interesting and useful.