On Monday, June 19, 2023, as part of the Colloquium "Sibe Mardešić" of the Department of Mathematics, the world-known Croatian and American mathematician prof. Mladen Bestvina, PhD, (University of Utah) held a lecture titled: “Mapping class groups: from Max Dehn to Bill Thurston and beyond”. Mladen Bestvina graduated in Zagreb, under the mentorship of Sibe Mardešić. In 1982, he went to the University of Tennessee, Knoxville, where he received his doctorate already in 1984 under the mentorship of John Walsh, solving the problem of topological characterization of multidimensional universal Menger compacts. He studied at Berkeley, Princeton and IHES. He held a position at the University of California, Los Angeles for nine years. Since 1993, he has been a full professor at the University of Utah in Salt Lake City, where he became a distinguished professor in 2008. Special recognition was given to him when he gave an invited lecture on the group of external automorphisms of a free group of rank n at the World Mathematical Congress in Beijing in 2002. In his scientific work, he made important contributions to geometric topology (universal continuums, manifolds, hyperbolic structures on 2- and 3-manifolds) and geometric group theory. Today, he is one of the leading specialists in this discipline in the world. Through his cooperation with mathematicians in Croatia, he significantly contributes to the progress of mathematics in our community. Since 2012, he has been a corresponding member of Croatian Academy of Sciences and Arts. This was the last lecture for this academic year as part of the Colloquium "Sibe Mardešić". Information about the lectures and lecturers can be found on the Colloquium's official website.