In this paper we describe the PhD Thesis Project "Impacto de los contenidos científicos en repositorios institucionales de acceso abierto. Un caso de estudio del repositorio Gredos de la Universidad de Salamanca" (Impact of Scientific Content in Open Access Institutional Repositories. A case study of the Repository Gredos of the University of Salamanca) into the PhD Programme called "Información y Documentación" (Information and Documentation) at the University of Salamanca. The working area of this research is the Open Knowledge. We explain that over the last decade the great development of the movement of Open Access has led to the creation of institutional repositories in order to store, disseminate and preserve digital documents of the institutions. Obviously, for researchers and institutions, it is essential to achieve visibility, citation and impact from their researching works. We study the relationship between the Open Access and the scientific impact of the papers. We focus on the PhD theses of the Salamanca University to demonstrate that the theses deposited in its institutional repository, called GREDOS (http://gredos.usal.es), have more citations and impact that the theses that are not Open Access. We introduce the design of the theoretical part and we explain the goals and the methodology we will use in this study.