This paper briefly reviews the standard Transmission Line Model (TLM) commonly used to measure the specific contact resistance ρ c and the sheet resistance Rsk beneath a planar ohmic contact. In the case of an alloyed ohmic contact, a more realistic three layer (Tri-Layer Transmission Line Model (TLTLM)) can be used for the analysis. This model is based on three layers (metal layer, alloyed semiconductor layer and the unalloyed semiconductor layer) and the two interfaces between them. By using appropriate TLTLM parameters, it is possible to calculate the sheet resistance Rsk that has been experimentally derived from the standard TLM. The new TLTLM model predicts that values of Rsk greater and less than Rsh (the unmodified epitaxial layer sheet resistance) are possible in agreement with experimentally reported observations.