Nanomedicine formulations aim to improve the biodistribution and the target site accumulation of systemically applied (chemo-) therapeutics. Various different passively and actively targeted nanomedicines have been evaluated over the years, based e.g. on liposomes, polymers, micelles and antibodies, and a significant amount of (pre-) clinical evidence has been obtained showing that these 5-200 nm sized carrier materials are able to improve the therapeutic index of low-molecular-weight drugs. Besides for therapeutic purposes, however, nanomedicine formulations have also been more and more used for imaging applications, as well as, in recent years, for theranostic approaches, i.e. for systems and strategies in which disease diagnosis and therapy are combined. Potential applications of theranostic nanomedicine formulations range from the noninvasive assessment of the biodistribution and the target site accumulation of low-molecular-weight drugs, and the visualization of drug distribution and drug release at the target site, to the optimization of strategies relying on triggered drug release, and the prediction and real-time monitoring of therapeutic responses. Nanotheranostic systems are consequently considered to be highly suitable systems for (pre-) clinical implementation, not only because they might assist in better understanding various important aspects of the drug delivery process, and in developing better drug delivery systems, but also because they might contribute to realizing the potential of "personalized medicine", and to developing more effective and less toxic treatment regimens for individual patients.