Magnetic nanoparticles for adsorption and subsequent magnetic removal of hazardous substances from water, as published in Science in 2006, bears enormous potential for water purification for which there is a growing need all over the world. Many publications followed this idea, but doubts remain whether nanoparticles are really separable and whether the process is really that simple. A closer look reveals uncertainties and the need for more research. (C) 2012 Elsevier Ltd. All rights reserved.