Suna Bensch (LPCN) and Henning Bordihn edited a special issue in the Journal of Automata, Languages and Combinatorics, dedicated to Professor Helmut Jürgensen on the occasion of his 75th birthday and in appreciation of his scientific work and his impact as teacher, mentor, and person. Among the sixteen papers in this special issue is the paper “The Output Size Problem for String-to-Tree Transducers” by Martin Berglund (LPCN), Frank Drewes (LPCN) and Brink van der Merwe.