Recent advances in nanotechnology have given rise to a new class of fluorescent labels, fluorescent metal nanoclusters, e.g., Au and Ag. These nanoclusters are of significant interest because they provide the missing link between atomic and nanoparticle behavior in metals. Composed of a few to a hundred atoms, their sizes are comparable to the Fermi wavelength of electrons, resulting in molecule-like properties including discrete electronic states and size-dependent fluorescence. Fluorescent metal nanoclusters have an attractive set of features, such as ultrasmall size, good biocompatibility and excellent photostability, making them ideal fluorescent labels for biological applications. In this review, we summarize synthesis strategies of water-soluble fluorescent metal nanoclusters and their optical properties, highlight recent advances in their application for ultrasensitive biological detection and fluorescent biological imaging, and finally discuss current challenges for their potential biomedical applications. (C) 2011 Elsevier Ltd. All rights reserved.