Nanotheranostics have recently attracted increasing attention for their simultaneous drug delivery and diagnostic imaging of diseases, particularly in cancer treatment. Among various nanomaterials, silica nanoparticles are one the most popular models for nanotheranostics. With a tailored mesoporous structure and high surface area, silica nanoparticles show significant advantages over traditional drug nanocarriers as drug delivery systems. With versatile silicone chemistry, silica nanoparticles can incorporate multiple components with optical, magnetic, and electrical properties for imaging and therapy. In this review, we give an overview of the recent progress in the synthesis and biomedical application of multifunctional silica nanoparticles for multimodal imaging, combination therapy, and the nanotheranostics of cancer. We also include our personal perspectives on the direction in which future work in this field might be focused.