Carbon nanomaterials and metal nanoparticles (NPs) are powerful agents for theranostics, which combines therapy and diagnostics, due to their diverse range of functionalities. Carbon nanostructures are functionalized with a range of bioactive compounds owing to their superior specific surface area, which also enables effective drug loading. Additionally, carbon derivates are excellent substrates for NPs attachment. Due to their optical and magnetic characteristics, these materials have also demonstrated significant promise for usage in the biomedical sector for medication administration, non-invasive cancer imaging, and early detection. In this regard, metal-based carbon nanocomposites display additional traits that result from interactions between the two entities in addition to adding distinctive properties of NPs and carbon compounds alone. The synthesis of metals and carbon nanomaterials and their uses in theranostics, imaging, treatment, and diagnostics are detailed in this chapter.