In an engineering context the precautionary principle is often perceived as an excuse to do nothing or a substantial barrier to technical progress. The precautionary principle requires that remedial measures be taken in situations of scientific uncertainty where evidence of harm cannot be proven but potential damage to human or environmental health is significant. In this paper the scope of the precautionary principle in water recycling is discussed. It is clear that uncertainties and risks exist in many areas of water recycling. These risks are closely linked to the risks of sewage discharge. Hence, water recycling has two main areas of concern: (1) the dilemma that minimising potential environmental harm by reducing effluent discharge may increase potential harm through reducing the water flow in receiving waters and (2) the consequences of using recycled water of varying quality for a number of applications. The precautionary principle can be regarded as an opportunity to improve water recycling practice and in fact increase the scope of ecologically sustainable water recycling. The precautionary principle has an important role to play as a guide in decision making and in dealing with the vast number of risks and uncertainties in water recycling.