The article discusses the fundamental origins and applications of spin-orbit interactions (SOI) in optics. Light, which carries both spin and orbital angular momentum, exhibits SOI phenomena that are crucial at subwavelength scales and in structured optical fields. Key examples include the spin-Hall effect in inhomogeneous media and at interfaces, spin-dependent effects in nonparaxial fields, and the use of anisotropic structures to control light polarization. The article also highlights the role of geometric phases in understanding these phenomena, emphasizing the interplay between spin angular momentum (SAM), intrinsic orbital angular momentum (IOAM), and extrinsic orbital angular momentum (EOAM). Additionally, it explores the unique properties of evanescent waves, which carry transverse spin, and their applications in spin-directional coupling. The review underscores the importance of SOI in modern optics, particularly in nano-optics, photonics, and plasmonics, where structured fields and subwavelength scales are essential.The article discusses the fundamental origins and applications of spin-orbit interactions (SOI) in optics. Light, which carries both spin and orbital angular momentum, exhibits SOI phenomena that are crucial at subwavelength scales and in structured optical fields. Key examples include the spin-Hall effect in inhomogeneous media and at interfaces, spin-dependent effects in nonparaxial fields, and the use of anisotropic structures to control light polarization. The article also highlights the role of geometric phases in understanding these phenomena, emphasizing the interplay between spin angular momentum (SAM), intrinsic orbital angular momentum (IOAM), and extrinsic orbital angular momentum (EOAM). Additionally, it explores the unique properties of evanescent waves, which carry transverse spin, and their applications in spin-directional coupling. The review underscores the importance of SOI in modern optics, particularly in nano-optics, photonics, and plasmonics, where structured fields and subwavelength scales are essential.