The Tvarminne Zoological Station (TZS) is a marine station located at the entrance to the Gulf of Finland in the Baltic Sea. As part of the Faculty of Biological and Environmental Sciences of the University of Helsinki, it serves as a centre for a variety of high quality biological and ecological research, carries out environmental monitoring and offers facilities for field courses, symposia and seminars. The station was established in 1902 and provides good background data for studying long-term environmental change. Biodiversity and its coupling to ecosystem level processes is a major research theme, involving pelagic and benthic habitats. These studies have contributed to understanding the structure and function of the Baltic Sea ecosystem and its unique characteristics.