The ‘speciation’ of metals is concerned with their different chemical and physical forms, a detailed knowledge of which is essential to understand what happens to metals in lakes and rivers, and their effects on living organisms. There have been many advances in recent years in our ability both to measure and predict metal speciation in natural waters, but it is not clear just how accurately these can be applied under field conditions. The current state of play is assessed to provide a better understanding of the related uncertainties, and suggestions are made for ways to reduce these uncertainties and possibly improve our predictive capabilities.