In Section 2.1, as a base for nanophotonics, we provide a quantum theoretical description of optical near fields and related problems that puts matter excitation such as electronic and vibrational ones on an equal footing with photons. With the help of the projection operator method, we derive effective interactions exerted in the nanometric material (nanomaterial) system surrounded by an incident light and a macroscopic material system, which are called optical near-field interactions. They are essential to understand the topics of the following sections, that is, the principles of operations of nanophotonic devices and those of nanofabrication using optical near fields.