Different methods for controlling the coupling between a two-conductor microstrip transmission line are investigated in this paper. The transmission line consists of two thin perfectly conducting strips with a rectangular notch between the strips, two layers of dielectric substrate, and a dielectric overlay. The formulation and solution technique are based on the equivalence principle, a free space Green's function, and the method of moments. It has been found that the coupling between the conducting lines can be reduced significantly if dielectric material between the lines is removed. Further reduction in coupling is observed when the relative permittivity of the upper substrate layer is made smaller than that of the lower substrate. Coupling can be maximized if the notch is filled with dielectric material having large relative permittivity compared to that of the substrates. If the strips are covered with a dielectric overlay (or superstrate), the coupling will be substantially increased. Moreover, with the right combination of dielectric material layers and the right geometry, it is possible to equalize the even- and odd-mode phase velocities while having some control over the coupling capacitance.