The present invention relates to methods of visualizing cells especially although not exclusively in vivo using a dye, such as a dendrimer-dye molecule or polybranched-dye molecule which is internalized by the cells and thus permits subsequent visualization by confocal fluorescence endomicroscopy or other optical detectors. There is also provided internally quenched probes for use in visualizing cells especially although not exclusively in vivo by confocal fluorescence endomicroscopy and the use of internally quenched probes in combination with confocal fluorescence endomicroscopy, for visualizing cells by virtue of internalization and dequenching of a probe by the cells. In a particular embodiment the cells are activated neutrophils, such as within the lung of a subject.