Physical optics (PO) is one of the fundamental and powerful high-frequency theories for electromagnetic scattering and radiation. The total field of a source (antenna) which radiates in the presence of a perfectly conducting surface may be expressed as a superposition of the incident and the scattered fields. The current fields which exist everywhere are chosen in PO to denote the electric and magnetic fields of the source, i.e., they exist as if the scatterer was “absent”; this is unlike the geometrical optics (GO) incident field, which exists in the presence of the surface of the scatterer. The scattered fields in this case can be expressed in terms of the radiation integrals over the actual currents induced on the surface of the scatterer. These currents also radiate the scattered fields in the absence of the scatterer. This chapter shows the fundamental PO formulation and calculated results, and some topics which improve the conventional PO to the extended PO such as “physical theory of diffraction (PTD)” and “PO with transition current (PTD-TC)”.