This volume contains the papers presented at the 30th Symposium on Mat- matical Foundations of Computer Science (MFCS 2005)held in Gdansk, Poland fromAugust29thto September 2nd,2005.TakingplacealternatelyintheCzech Republic, Slovakia and Poland, this year the conference was organized by the Institute of Mathematics of Gdansk University. From the ?rst meeting in 1972 to this year s 30th event, the MFCS series has provided a basis for theoretical computer scientists to present their latest research results. The scope of the conference, consequently, covers all branches of theoretical computer...