Two gas pipelines connecting Russia and Germany through the Baltic Sea are planned to be constructed during the period 2010–2012. The pipeline system will enable an annual transportation of 55 billion cubic metres (bcm) of natural gas from Russia to the European gas grid. Comprehensive studies of the potential environmental impacts of the project have been carried out. Special attention has been given to the fact that the Baltic Sea is a unique and vulnerable environment. It can be considered the world's largest estuary, with marine life adapted to the brackish water environment and with limited water exchange with the surrounding sea through the Danish straits. Valuable experience from major bridge and tunnel construction works across the Danish straits during the 1990s has been used in order to carry out an assessment of the environmental impact of the gas pipeline project and in order to design the construction works in an environmentally friendly way. The environmental aspects of the various phases of the pipeline project have been identified, and their environmental significance has been assessed. A rating system for classifying the environmental impacts has been developed. Classification has been carried out based on type of impact and extent in terms of space and time. The environmental aspects of the project have been integral to the planning phase to ensure that the design of the pipeline system is environmentally optimised. A major task has been to identify the optimal route, taking into account both the important nature areas in the Baltic Sea and the special precautions related to the presence of dumped munitions in parts of the Baltic Sea. The environmental analyses show that the pipelines can be constructed, operated and decommissioned with no long-term impacts on the marine life of the Baltic Sea. Introduction According to present estimates, the EU's annual natural gas requirement of 550 bcm in 2006 will rise by approximately 130 bcm to approximately 680 bcm per year in 2020. At the same time the production of gas in the EU is expected to decline to cover only 20% of its projected natural gas requirement in 2020. In order to meet future energy demands in the EU, the EU Commission has launched the Trans-European Energy Networks (TEN-E) programme. The Commission particularly suggests expansion of the EU's supply relationship with Russia. The Nord Stream project, which consists of two gas pipelines at the bottom of the Baltic Sea connecting Russia with Germany, has been designated as an EU priority project within the TEN-E programme. In Fig. 1, the planned Nord Stream pipeline route (represented by the blue dotted line) from Russia to Germany is shown. The brown areas represent major gas-bearing geological areas in the region, and the brown lines represent the existing gas transport infrastructure in northern Europe. The Nord Stream pipeline is a response to the fact that the major gas fields in northern Europe are situated in Russia and offshore on the Russian and Norwegian continental shelves, whereas the major energy consumption takes place in western Europe. In particular, the Shtokman Field in the Russian part of the Barents Sea is expected to supply gas through the Nord Stream pipelines.