Fluorescence imaging techniques are becoming essential for preclinical investigations, necessitating the development of suitable tools for in vivo measurements. Nanotechnology entered this field to help overcome many of the current technical limitations, and luminescent nanoparticles (NPs) are one of the most promising materials proposed for future diagnostic implementation. NPs also constitute a versatile platform that can allow facile multi-functionalization to perform multimodal imaging or theranostics (simultaneous diagnosis and therapy). In this contribution we have mainly focused on dye doped silica or silica-based NPs conjugated with targeting moieties to enable imaging of specific cancer cells. We also cite and briefly discuss a few non-targeted systems for completeness. We summarize common synthetic approaches to these materials, and then survey the most recent imaging applications of silica-based nanoparticles in cancer. The field of theranostics is particularly important and stimulating, so, even though it is not the central topic of this paper, we have included some significant examples. We conclude with a short section on NP-based systems already in clinical trials and examples of specific applications in childhood tumors. This review aims to describe and discuss, through focused examples, the great potential of these materials in the medical field, with the aim to encourage further research to implement applications, which today are still rare.