The nanomaterials bear considerable potential as the next generation of therapeutic vehicles that allow the early detection of disease and the simultaneous monitoring and treatment options for targeted drug delivery with reduced toxicity. Carbon-based nanomaterials are in consideration in several clinical areas. It is due to the intrinsic properties, i.e., nanosize, distinctive structural dimensions, and diversified physicochemical properties, which attract the scientific communities for their applications in the drug delivery area as well as for the development of novel diagnostic assays with high specificity and sensitivity. Structurally, these carbon-based nanomaterials exhibit zero-, one-, two-, and three-order dimensions with several valuable features that could be exploited potentially in theranostic nanomedicines. The term "theranostic" was recently implemented for the two-fold beneficial application of the materials simultaneously, as a therapeutic entity and for the diagnostic purpose. The current chapter describes the applications of carbon-based nanomaterials in drug delivery and disease diagnosis. The synthesis of carbon-based nanomaterial and functionalization approaches and possible future theranostic applications are also included.