  Light carries spin and orbital angular momentum. These dynamical properties
are determined by the polarization and spatial degrees of freedom of light.
Modern nano-optics, photonics, and plasmonics, tend to explore subwavelength
scales and additional degrees of freedom of structured, i.e.,
spatially-inhomogeneous, optical fields. In such fields, spin and orbital
properties become strongly coupled with each other. We overview the fundamental
origins and important applications of the main spin-orbit interaction phenomena
in optics. These include: spin-Hall effects in inhomogeneous media and at
optical interfaces, spin-dependent effects in nonparaxial (focused or
scattered) fields, spin-controlled shaping of light using anisotropic
structured interfaces (metasurfaces), as well as robust spin-directional
coupling via evanescent near fields. We show that spin-orbit interactions are
inherent in all basic optical processes, and they play a crucial role at
subwavelength scales and structures in modern optics.
