This paper reviews several applications of photonic side bands, used by Buttiker and Landauer (Phys. Rev. Lett. 49, 1739 (1982)) in their theory of traversal time in tunneling, in transport and optics of mesoscopic systems. Topics include generalizations of the transmission theory of transport to time-dependent situations, optics and transport of mesoscopic systems in THz electromagnetic fields, and phase-measurements of photon-assisted tunneling through a quantum dot. (C) 1998 Academic Press Limited.