This volume contains papers selected for presentation at the 26th International Symposium on Mathematical Foundations of Computer Science - MFCS 2001, held in Mari'ansk'eL'azn?e, Czech Republic, August 27 - 31, 2001. MFCS 2001 was organized by the Mathematical Institute (Academy of S- ences of the Czech Republic), the Institute for Theoretical Computer Science (Charles University, Faculty of Mathematics and Physics), the Institute of C- puter Science (Academy of Sciences of the Czech Republic), and Action M Agency. It was supported by the European Research Consortium for Informatics and Mathematics, the Czech Research Consortium for Informatics and Ma- ematics, and the European Association for Theoretical Computer Science. We gratefully acknowledge the support of all these institutions. The series of MFCS symposia, organized on a rotating basis in Poland, S- vakia, and the Czech Republic, has a well-established tradition. The aim is to encourage high-quality research in all branches of theoretical computer science and bring together specialists who do not usually meet at specialized confer- ? ences. Previous meetings tookplace in Jablonna, 1972; Strbsk'e Pleso, 1973; J- wisin, 1974; Marian ' sk'eL'azn?e, 1975; Gdan 'sk, 1976; Tatransk'a Lomnica, 1977; Za- ? kopane, 1978; Olomouc, 1979; Rydzina, 1980; Strbsk'e Pleso, 1981; Prague, 1984; Bratislava, 1986; Karlovy Vary, 1988; Porabk , a-Kozubnik, 1989; Bansk'aBystrica, 1990; Kazimierz Dolny, 1991; Prague, 1992; Gdan 'sk, 1993; Ko?sice, 1994; Prague, 1995; Krak'ow, 1996; Bratislava, 1997; Brno, 1998; Szklarska Por,eba, 1999; and Bratislava, 2000.
