B. Helffer, M. Hoffmann-Ostenhof, T. Hoffmann-Ostenhof, M. Owen Nodal sets, multiplicity and superconductivity in non simply connected domains (499K, Postscript) ABSTRACT. This is a survey on a recent paper of the present authors (CMP 1999). We explain in detail the origin of the problem in superconductivity as first presented by Berger and Rubinstein (CMP 1999), recall our results and explain the extension to the Dirichlet case. As illustration of the theory, we detail some semi-classical aspects and give examples where our results are sharp.