Abstract: Nanostructures enable amplitude and phase manipulation of light at the subwavelength scale. The renewed and ever-increasing interest in nanoplasmonic and nanophotonic research is motivated by the rapid advancements in nanofabrication technologies and characterization tools, by the easier access to powerful and fast computational resources and by the progress in related research fields, e.g., metamaterials science. The existing and potential applications of nanostructures are innumerable and span a wide range of fields, including energy harvesting, sensing devices, electro-optical devices, biomolecule detection, drug delivery, and many more. We will discuss linear and nonlinear optical phenomena and we will give some examples of sensing and imaging devices based on nanostrutures.