The paper starts with the assumption that the Precautionary Principle (PP) is one of the most important elements of the concept of sustainability. It is noted that PP has entered international treaties and national law. PP is widely referred to as a central principle of environmental policy. However, the precise content of PP remains largely unclear. In particular it seems unclear how PP relates to science. In section 2 of the paper a general overview of some historical and systematic features (...) of PP are presented. In section 3 a specific case is discussed in greater detail. It is claimed that the escape of farmed salmon from fish cages in the Sea, and its eventual invasion of the breeding places of the wild salmon up the rivers, must be regarded a proper case for applying PP. Yet there is no single PP-strategy. Instead, four different strategies are presented, and all of them can be regarded precautionary strategies in the light of PP. The choice between these strategies is based upon personal values. In section 4 of the paper a general analysis is given which relates these different value perspectives to basic differences in risk aversion, which in turn are related to differing conceptions of nature and/or society. In the concluding section 5 some general consequences of the foregoing analysis are outlined. (shrink)