Carbon-carbon allotropic hybrids and their composites are reviewed. Carbon-carbon hybrids are mainly composed of graphene, graphite, graphene oxide, reduced graphene oxide, carbon nanotubes, carbyne chains, fullerenes and more complex spherical and ring-ike carbon forms, graphene quantum dots, and carbon nanodots. Aerogels and xerogels on their basis are also discussed. Most of these hybrides consist of 3D architectures with either covalent or van der Waals interactions between carbon atoms, possessing unusual properties as compared to their counterparts, because of the simultaneous presence of carbon structures of distinct dimensionality and reactivity. These composites can be prepared by a variety of methods starting from already existing allotropes, or by their synthesis in situ, such as chemical vapor deposition, solvothermal techniques, pyrolysis, ultrasonication in solution, and other liquid-phase methods, frequently including redox steps. Current and potential applications of hybrids and their metal-doped composites as supercapacitors, sensors, catalysts, and environmental remediation reactants are described.