Whole-cell biosensors are finding increasing use for the detection of environmental pollution and toxicity. These biosensors are constructed through the fusion of promoters, responsive to the relevant environmental conditions, to easily monitored reporter genes. Depending on the choice of reporter gene, expression can be monitored by the production of colour, light, fluorescence or electrochemical reactions. Recent advances in this area have included the development of biosensors of compact size that enable the on-line and in situ monitoring of a large number of environmental parameters.