The theory of a free string is first presented classically and quantum mechanically both in a covariant and a nonexplicitly covariant treatment. Then the concepts and techniques obtained from the free string are used to build the operator formalism of dual models. Both the conventional Veneziano model and the Neveu-Schwarz-Ramond model are presented. The self-consistency of these models at the tree and loop levels is investigated, and the interpretation of the results obtained is given in terms of the string picture.