This book introduces the central theses of Karl-Otto Apel's philosophy, which are often scattered over many texts of his work. His work is made transparent in the field of tension between the major debates between Habermas' critical theory and the philosophy of critical rationalism. Walter Reese-Schäfer often makes Apel's difficult language understandable, names the prerequisites of discourse ethics and defines their scope. Thus, this introduction offers indispensable help for all those who are looking for an initial, but also in-depth approach to his work and to the ethics of discourse.