The ability to find synergic solutions is the core of scientific research and scientific advancement. This is particularly true for medicine, where multimodal imaging and theranostic tools represent the frontier research. Nanotechnology, which by its very nature is multidisciplinary, has opened up the way to the engineering of new organized materials endowed with improved performances. In particular, merging nanoparticles and luminescent signalling can lead to the creation of unique tools for the design of inexpensive, hand-held diagnostic and theranostic kits. In this wide scenario, dye-doped silica nanoparticles constitute very effective nanoplatforms to obtain efficient luminescent, stable, biocompatible and targeted agents for biomedical applications. In this review we discuss the state of the art in the field of luminescent silica-based nanoparticles for medical imaging, starting with an overview of the most common synthetic approaches to these materials. Trying to rationalize the presentation of this extremely multifaceted and complex subject, we have gathered significant examples of systems applied in cancer research, also discussing those that take a multifunctional approach, including theranostic structures. Nanoprobes designed for applications that do not include cancer are a minor part, but interesting achievements have been published and we present a selection of these in the subsequent section. To conclude, we propose a debate on the advantages of creating chemosensors based on luminescent silica nanoparticles. This is far from easy but is a particularly valuable goal in the medical field and therefore subject to extensive research worldwide. [ABSTRACT FROM AUTHOR]