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 science ranging from automata, algorithms, data str- tures, models of computation to complexity theory, also including arti?cial - telligence, computational biology, computational geometry and cryptography. The 137 submissions from 22 countries revealed a continued strong interest in the conference as well as the high-quality research results the MFCS series stands for. The Program Committee carefully selected 62 papers for presention at the conference complemented by 7 invited talks. The meeting took place at a conference hotel located on Sobieszewo Island, 15 km from Gdansk,o?ering both a beautiful landscapewith sandy beaches and forests and the possibility to explore the old Hanseatic city of Gdansk with its interesting history of over 1000 years.