The authors present a systematic development of the concurrent object-oriented agent programming language Guarded Definite Clauses (GDC). In contrast to other languages used in agent programming, like Java, Telescript, and Agent-TCL, this language is derived from the artificial intelligence programming tradition and emphasizes AI applications. The first part of the book is devoted to the principled evolution of the paradigm GDC; during the course of this evolution, the reader can also learn a lot about the history and the dramatically changing fortune, booms, and busts, of AI. In the second part, the paradigm is evaluated for application in various fields including parallel distributed search, distributed constraint solving, meta-interpretation, partial evaluation, and robotics and multi-agent systems. The book is written for students and professionals in agent programming or in AI programming in general.