The goal of data integration is to provide programs and human users with integrated access to multiple, heterogeneous data sources, giving each user the illusion of a database, designed for single homogeneous specific needs her. This book is an introduction to the problem of data integration and rigorous account of one of the main approaches to solve this problem, namely, relational logic approach. relational logic provides a theoretical framework to discuss the integration of data. In many ways, the logic is not relational algebra for the integration of data what relational databases did for the theory a few decades ago.