There has been growing interest in recent years in reinforcing polymers with different types of nanostructures to produce nanocomposites. At the same time there have been new developments in the field of carbon-based nanomaterials with the discovery of carbon nanotubes and graphene with impressive mechanical properties. The presentation will show how Raman spectroscopy can be used to characterize these carbon-based nanomaterials and in particular how it can be employed to follow their deformation in nanocomposites from stress-induced Raman bands shifts. The aim of this presentation is to determine how stress transfer takes place both between the nanomaterials and the polymer matrix and between the different carbon layers within the reinforcement. In particular, the mechanics of the deformation of single-, double- and multi-walled carbon nanotubes will be compared and it will be shown that inter-wall stress transfer by shear between the individual walls of the nanotubes has an important effect upon controlling their ability to reinforce polymers. Large stress-induced Raman bands shifts are found during deformation as a result to stress transfer from the matrix to the nanotubes. Band broadening is found during deformation with the single-walled nantoubes whereas it is found that band narrowing occurs for the double- and multi-walled nanotubes. This has been interpretated as being due to lower levels of stress in the inner walls as the result of poor internal stress transfer and the behavior has been modelled fully. In general it is found that this internal stress transfer is only up to 70% efficient leading to inferior mechanical properties for double- and multi-walled nanotubes in nanocomposites compared with their single-walled counterparts.