Cavity-backed slot antennas have been widely used in different types of applications within the microwave band, including radar, satellite communications, mobile telephony, broadcast TV, and aircraft/spacecraft communications. They are relatively easy to manufacture, light-weight and often small in size. Their low profile is an important characteristic, especially for aircraft, missile and spacecraft, applications, because they can be flush-mounted on the surface of airborne vehicles without affecting the vehicle's aerodynamic profile. In this paper, the coupling of cavity-backed slot antennas is analyzed by using the finite-difference time-domain (FDTD) method and the finite element method (FEM). Also, parametric studies are performed to examine the dependence of the coupling on the frequency of operation and the separation between the apertures. The numerical results are validated by comparison with measurements.