Organizational knowledge and its different forms play a critical role in order to effectively develop the products and processes of the firm (Kogut and Zander, 1996; Teece, 1998; Ategre and Lapiedra, 2005). This work explores the influence of the different forms of organizational knowledge (or Intellectual Capital) on the innovation capability of professional service firms. For reaching this purpose, in a first section, an exploratory factor analysis is earned out, obtaining the three main blocks of Intellectual Capital: Human Capital, Structural Capital, and Relational Capital. In the next step, the basic hypothesis about the positive influence of these kinds of Intellectual Capital (and their main dimensions) on firm innovation, are presented. From the empirical findings obtained for Spanish professional service firms, it is special worthwhile to highlight the role of customer relationships, as well as IT-based competences, alliances, and corporate reputation. [ABSTRACT FROM AUTHOR]