We describe the preparation of very bright and photostable fluorescent silica nanoparticles (SiNPs), with emission in the visible or in the near infrared (NIR). The nanoparticles can be prepared with diameters of 30-300 nm with low size dispersity, and incorporate perylenediimide (PDI) dyes modified with alkoxysilane groups that react with the silica structure. One of the dyes (PDIvis) emits in the green and the other (PDInir) in the NIR (with fluorescence quantum yields of 90% and 24%, respectively). Silica nanoparticles containing these dyes were efficiently internalized in HEK293 cells with low toxicity and reveled very high photostability, showing good potential for use as a platform for in vivo laser scanning bioimaging applications. The NIR emitting nanoparticles, in particular, can be used in multi-color imaging, even in cells expressing high levels of fluorescent proteins and/or co-stained with different fluorescent dyes (most of which commonly emit at wavelengths lower than the NIR).