The present selection from the Wissenschaftslehre (Sulzbach 1837) of Bernard Bolzano (1781-1848) aims at giving a compact view of his main ideas in logic, semantics, epistemology and the methodology of science. These ideas are analyzed from a modern point of view in the Introduction. Furthermore, excerpts from Bolzano's correspondence are included which yield important remarks on his own work. The translation of the sections from the Wissenschaftslehre are based on a German text, which I have located in the Manuscript Department of the University Library in Prague (signature: 75 B 459). It was one of Bolzano's own copies of his printed work and contains a vast number of corrections made by Bolzano himself, thus representing the final stage of his thought, which has gone unnoticed in previous editions. The German originals of Bolzano's letters to M. J. Fesl, J. P. Romang, R. Zimmermann and F. Pi'ihonsky are in the Literary Archive of the Pamatnfk narodnfho pfsemnictvf in Prague. The original of the letter to F. Exner belongs to the Manuscript Department of the Osterreichische Nationalbibliothek in Vienna. The original of the letter to J. E. Seidel is preserved in the Museum of the City of Ceske Budejovice.