Near-Infrared (NIR) dyes forming some of the photosensitizer agents show imaging and therapy features by themselves. NIR dyes show photodynamic therapy by formation of reactive oxygen species and imaging by NIR Fluorescence light. Photodynamic therapy occurs from irradiation of laser or light to photosensitizer matters and following by the formation of reactive oxygen species diseased tissues or cells can be killed effectively. NIR dyes have advantages such as stability, high specificity and sensitivity when compared with the other photosensitizer and imaging agents. Drug delivery systems are getting attention for either diagnosis or therapy of almost all of the diseases. Theranostic nanoparticles comprise the substances which shows the imaging and treatment features together. Besides, the combination of active substance and the imaging agent can also be encapsulated in theranostic nanoparticles. Many researchs are performed to evaluate the efficacy of theranostic drug delivery systems particularly polymeric nanoparticles in order to enhance targeting properties, specificity and bioavailability. Polymeric nanoparticles give advantages because of easier degradation properties when compared with the others. Theranostic polymeric nanoparticles can be used for NIR imaging and photodynamic therapy of several diseases especially cancers.