Lake Baikal in east Siberia contains more than 20% of the available fresh water on Earth. It represents the world's largest (23,000 km3), deepest (1,630 m) and oldest (> 20 My) liquid fresh water body. The water of Lake Baikal is very clean and of perfect quality. It can be used as drinking water without treatment of any kind. In 1989 the European members of the World Health Organisation signed the European Charter on Environment and Health which identifies the restoration and protection of surface waters as a top-priority aim. This goal is of paramount importance for Lake Baikal as this lake simultaneously provides the economic base for the local population (3 million), represents a fresh water resource of global importance and is the home of a unique aquatic ecosystem with more than 1,000 endemic species. The importance and fragility of this lake system makes it urgent to study and understand the physical, chemical and trophic mechanisms determining its dynamics and to assess the relative role of natural versus anthropogenic changes occurring in the system.