In March 2010 the Digital Library of the Wielkopolska Region, the largest and one of the oldest Polish digital librariesreached the level of 100 000 objects available on-line. Since its inauguration in 2002, the dynamics of the development of Polish digital libraries has increased significantly. The common effort of several hundreds of scientific and cultural institutions in Poland led to the present situation in which there are over 50 publicly available digital libraries and together they give access to around 425 000 digital objects. Such a big number of similar network services lead to the need of integration mechanisms allowing to achieve a synergy effect perceptible for end users as well as for digital library creators and international digital library initiatives. Suchmechanism developed by PSNC and named PIONIER Network Digital Libraries Federation, has been available as a network servicesince June 22-nd, 2007 and can be reached at http://fbc.pionier.net.pl/. This paper shortly presents the history of development ofPolish digital libraries, showing the factors that have influenced the initial form and advancement of the Federation. It also describesthe Federation architecture, organizational aspects of its operation, and key challenges including the integration of Polish and European digital libraries infrastructures.