Open strings and boundary fermions

This paper will be organized as follows: First I will give a general reminder of what open strings are. Next I will describe the introduction of Chan-Paton factors, and their rewriting as the quantum mechanics of fermions on the boundary of the world sheet. Then I will describe how these fermions can be used to construct sigma models describing non-abelian Yang-Mills fields, both in the bosonic and supersymmetric open string theories, and I will do some illustrative calculations in these sigma models. Finally I will end with a few conclusions. 23 refs., 7 figs