Book excerpt: Off-shell science deals with the quantum field in which the dispersion relation between energy and momentum is invalid. A typical example of such the quantum field is the dressed photon (DP) that creates by the interaction among photons, electrons, and phonons in a nano-particle. This field is complementary to the on-shell quantum field (photons in a macroscopic space). Off-Shell Applications in Nanophotonics: Dressed Photon Science and Technology reviews the experimental/theoretical studies and shows the route that should be taken to establish off-shell science in the future. A variety of phenomena originate from the DP, and phenomena analogous to them have been found among physical, chemical, and biological phenomena. This indicates that off-shell quantum fields are universal and essential constituent elements of nature. By noting this, readers will be able to use off-shell science to develop new technologies. This book presents i) the reasons why the off-shell scientific theory is required, ii) the nature of the dressed photon by presenting experimental results, iii) tentative theoretical description of the dressed photon, iv) disruptive innovations (nano-optical devices, nano-fabrication technology, energy conversion technology, and silicon light-emitting diodes/lasers), and v) genuine theoretical approaches (based on spatio-temporal vortex hydrodynamics, quantum probability, quantum measurement, and micro-macro duality). It will appeal to materials scientists, engineers and physicists working in the areas of optics and photonics. - Explains the fundamental concepts behind off-shell science, and how it differs from traditional nanophotonics - Presents a range of simulation models demonstrating major off-shell models - Assesses the major challenges for researchers wanting to employ off-shell-based experimental techniques