Hybrid nanostructures of metal, polymer, chalcogenide, oxide, hydroxide, nitride, and carbonaceous materials are believed to promise a great impact on the next generation of cancer diagnosis and therapy technologies. This chapter focuses on hybrid nanostructures that contain two or more distinct nanoparticles assembled in a functional structure that itself is still of nanoscale dimensions. The aim of this type of research is to build a hybrid nanostructure whose medical effects are superior to those that could be realized from any simple mixture of the individual nanostructures. The unique characteristics of hybrid nanostructures in the nanometer range, such as high surface-area-to-volume ratio or shape/size-dependent optical properties, are drastically different from those of their bulk counterparts and hold pledge in the clinical field for disease therapeutics. This chapter deals with various kinds of novel hybrid nanostructures such as TiO2@Au, nanotube, silica, gold, and polymer that have demonstrated potential biogenic applications, especially in cancer research, including detection and treatments. Synergistic effects corroborated in these hybrid organic and inorganic nanostructures, useful for theranostics, are also explored at the end of the chapter.