The last comprehensive biography on Francesco Filelfo was written well over one hundred and fifty years ago. Since then the general state of knowledge about this humanist has been largely conditioned by G. Voigt's hostile assessment and G. Bendaucci's unsystematic and unreliable studies. Monographs on Filelfo's stay at Florence and Siena have been provided by G. Zippel and L. de Feo Corso, but the chief period in Filelfo's life, i.e. Filelfo at the court of Milan, has so far not been studied in adequate depth. E. Garin's recent account of Filelfo at Milan does not open up any new vistas. Yet Milan was the city where Filelfo spent half his life, where he wrote almost all his works and where he left a deep imprint in the development of humanistic culture. This thesis is therefore intended to fill this gap. The recent publication of P.O. Kristeller's 'Iter Italicum' made it possible to base such a reappraisal on an extensive survey of Filelfo manuscripts in Italian libraries. Almost all the existing Filelfo manuscripts at Rome, Florence, Milan, Pisa, Lucca, Bergamo, Venice, Munich, Oxford, Holkham Hall and London have been examined for this thesis. All unpublished material found there had to be copied and editions had to be prepared. Only Vienna, Paris and Wolfenbüttel seem to hold still unknown works. Particularly in the archives of Florence and Milan a large amount of entirely new material has been discovered which is being edited for the first time in the appendix of this thesis. It throws a significant light on Filelfo's social and economic situation. It allows us to penetrate the curtain of rhetorical declamations of Filelfo's letters and to understand the economic and cultural reality that lay behind them. Another purpose of this thesis consisted in the compilation of a bibliography in which all the various publications on Filelfo since about 1870 are listed, for they are scattered in periodicals and sometimes difficult to trace. [Continued in text ...]