The use of terahertz (THz) imaging as a modality of medical imaging techniques for diagnosing cancers is feasible. However, it cannot be used for clinical trials because of its low sensitivity, which is insufficient for clearly differentiating cancerous tumors from healthy tissues, and some other drawbacks. These problems can be solved by using nanoparticle (NP) probes as contrast agents for THz imaging. NP probes can be delivered to or targeted toward tumors, organs, or cells by various techniques. When NPs are irradiated with a near-infrared (NIR) laser beam, a cell’s ambient temperature increased because of the resonance of surface plasmon polaritons. This temperature rise changes the optical properties of the cell’s water, e.g., the absorption and refractive index much at THz frequencies because water molecules show a characteristic resonance at frequencies around 6 THz. Therefore, the THz absorbance and reflectance change dramatically by the NIR irradiation of NPs. On the basis of this principle, molecular imaging using THz waves was demonstrated, and factors such as sensitivity, resolution, and quantification property were characterized. As an example of medical diagnosis, a THz molecular imaging (TMI) technique was used for characterizing cancerous tumors and measuring the distribution of NP drug delivery to organs in vivo and ex vivo.