Biological sensing using nanoparticles Colloidal fluorescent and plasmonic nanoparticles yield intense responses to incident light, making them useful as sensors or probes for sensitive detection in solution. Howes et al. review the potential uses of nanoparticle biosensors in research and diagnostics. A range of methods allow for the chemical modification of the particle surfaces so that they can be tuned for specific analytes and give optical signals for a range of biological conditions of interest. Signals can be detected in complex media or in vivo making the particles of interest for both laboratory research and in clinical settings. Science , this issue 10.1126/science.1247390