This volume is the first in a series which will, in the future, be published jointly by Bibliopolis, Edizioni di Filosofia e Scienze spa, Napoli, and under the North-Holland imprint by Elsevier Science Publishers, Amsterdam. This is the first of a two-part work covering the main ideas and techniques of proof theory. This volume is introductory, starting with Hilbert (whose second problem in the famous list of problems of 1900 was the consistency of number theory), describing Hilbert's program and its demise at the hands of Godel. It then proceeds with Gentzen's result (the Hauptsatz is the main result of Part I) and improvements and generalizations to omega-logic. Volume II will cover more advanced logics.