In this paper, we review two main recently dominating applications of magneto-optics (MO). The first one is related to a unique MO non-reciprocity. For example, the MO non-reciprocity in the isolators enables complete transmission in the forward propagation direction, while it prevents spurious back-reflection, which is needed to preserve proper operation of active optical elements like lasers or amplifiers in optical systems. Local enhancement of MO activity by optical field concentration in nanostructured magneto-plasmonic and magneto-photonic systems opens new horizons in optical isolators, circulators, and switches. We will discuss enhancement of MO effects using surface magneto-plasmons in periodic grating and apply it to nonreciprocal isolating systems. The second main application of the magneto-optics is the characterization of magnetic multilayers, periodic systems, and nanostructures. MO techniques profit from high near-surface sensitivity to local magnetization, nondestructive character, ultrafast response, and possibility to measure all components of the magnetization vector by means of MO vector magnetometry. Furthermore, the MO Kerr effect allows the separation of magnetic contributions originating in different depths, different materials in multilayer systems as well as laterally modulated and self-organized nanostructures fabricated via modern nanotechnologies.