Conformal antennas, used extensively on mobile communications systems, generally have complex configurations and are often times mounted on or used within large structures. Hence, the modeling of such antennas is a challenging problem. The most frequently used numerical techniques either require excessive computational resources or cannot effectively model such structures. Thus, hybrid techniques have recently been employed to take advantage of each method''s strengths, while minimizing weaknesses. To enhance computational efficiency, a new methodology is introduced in this thesis, based on a combination of the Finite Difference Time Domain (FDTD) and the Method of Moments (MoM) numerical techniques in conjunction with the Surface Equivalence and Reciprocity Theorems. Several antenna configurations are considered to illustrate the new methodology: 1) radiating slots on conducting cylinders, 2) microstrip patch antennas mounted on large conducting structures, and 3) reflector antennas with partial circular symmetry.