Light can carry both spin and orbital angular momenta, which are determined by light’s polarization and spatial degrees of freedom respectively. In paraxial fields, optical spin and orbital angular momenta are mutually independent and conserved under free-space propagation. By contrast, in nonparaxial fields, such as tightly focused or scattered fields, the coupling and transformation between the spin and orbital angular momenta occur. Particularly, because it is widely involved in applications such as optical trapping, microscopy and sensing, spin-orbit interactions (SOIs) occurring in tight focusing systems have gained much attention recently. This review presents the theoretical calculation of spin and orbital angular momenta in the tightly focused fields, reveals the relationship between SOIs and the incident structured light fields, and briefly introduces the latest progress of the related applications.