The Natural Step (TNS) is internationally promoted as the basis of a scientific consensus on sustainability. TNS appears potentially consensual because it argues from a position of summary trends that are scientifically informed. However, comparison with other sustainability principles shows that this appearance is unjustified. Firstly, this is because sustainability principles vary widely in their implications and purpose. Secondly, TNS is ambiguous in its approach to risk assessment and controversial in its implied proposal for zero growth in the physical parameters of the human economy, biodegradable material excepted. Two concepts from the sociology of science are used to account for international corporate and public uptake of TNS, despite its ambiguity and highly precautionary message. These are the boundary object and anchoring devices. TNS illustrates how operationalization of sustainability theories unavoidably involves value judgements relating to the choice of features to be sustained, despite any scientific content that those theories may have. Copyright (C) 2000 John Wiley and Sons, Ltd and ERP Environment.