Singular integral operators play the central part in modern harmonic analysis. Simplest examples of singular kernels are given by Calderón-Zygmund kernels. Many important properties of singular integrals have been thoroughly studied for Calderón-Zygmund operators. In the '80s and early '90s, Coifman, Weiss, and Christ noticed that the theory of Calderón-Zygmund operators can be generalized from Euclidean spaces to spaces of homogeneous type. The purpose of this book is to make the reader believe that homogeneity (previously considered as a cornerstone of the theory) is not needed. This claim i.