Singularities of the minimal model program This book gives a comprehensive treatment of the singularities that appear in the minimal model program and in the moduli problem for varieties. The study of these singularities and the development of Moris program have been deeply intertwined. Early work on minimal models relied on detailed study of terminal and canonical singularities but many later results on log terminal singularities were obtained as consequences of the minimal model program. Recent work on the abundance conjecture and on moduli of varieties of general type relies on subtle properties of log canonical singularities and conversely the sharpest theorems about these singularities use newly developed special cases of the abundance problem. This book untangles these interwoven threads presenting a self contained and complete theory of these singularities including many previously unpublished results In 1982 Shigefumi Mori outlined a plan now called Moris program or the minimal model program whose aim is to investigate geometric and cohomological questions on algebraic varieties by constructing a birational model especially suited to the study of the particular question at hand. The theory of minimal models of surfaces developed by Castelnuovo and Enriques around 1900 is a special case of the 2 dimensional version of this plan. One reason that the higher dimensional theory took so long in coming is that while the minimal model of a smooth surface is another smooth surface a minimal model of a smooth higher dimensional variety is usually a singular variety. It took about a decade for algebraic geometers to understand the singularities that appear and their basic properties. Rather complete descriptions were developed in dimension 3 by Mori and Reid and some fundamental questions were solved in all dimensions