Strain gauges are devices whose electrical resistances vary proportionately with the amount of strain applied on
the device. They can be used for real-time applications in the aerospace sector, as a geotechnical tool in tunnels and bridges,
in rail monitoring and health monitoring sectors. Nanomaterials have been widely used for this application because they can
be flexible, stretchable and have high strength. Several researchers have used numerous carbon-based and metallic
nanostructures to develop functionally graded materials. Among carbon-based materials, graphene has been widely researched as a viable material for strain sensors due to its superior mechanical and electrical properties. Also, many metallic
nanoparticles have been investigated to design strain sensors that are highly sensitive at a wide range of strains. In this
article, a review of carbon and metallic nanomaterial-based strain sensors is presented, with emphasis on applications pertaining to structural health monitoring and wearable devices.