This special issue is 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. The sixteen papers in this special issue were submitted by invitation of the guest editors. Each paper was reviewed by at least two referees. The authors of the papers in this special issue are collaborators, co-authors, or scientific descendents of Helmut Jürgensen.