Despite recent progress in the treatment of cancer, the majority of cases are still diagnosed only after tumors have metastasized, leaving the patient with a grim prognosis. However, there may be an opportunity to drastically reduce the burden of cancer, if the disease can be detected early enough. Nanotechnology is in a unique position to transform cancer diagnostics and to produce a new generation of biosensors and medical imaging techniques with higher sensitivity and precision of recognition. This review examines the in vitro and in vivo diagnostic applications of nanoparticles, and other nanodevices that are likely to have an impact on the field in the future. Future developments that may lead to the realization of multifunctional detection and treatment nanoparticle platforms are also discussed.