This study measured the photon beam attenuation characteristics of the Varian EXACT, Varian IGRT treatment couches and two CIVCO Universal extensions used during external beam radiotherapy; and assessed the accuracy of Varian's Eclipse treatment planning system in modeling the beam attenuation of each couch. Dose Measurements were taken using a Farmer ion chamber inside a cylindrical acrylic phantom. They were taken for two photon energies (6MV and 15MV), at three different field sizes, for various gantry angles through the couch. The EXACT couch was tested both with the adjustable rails pushed to the outside of the couch, and pushed to the center of the couch. The IGRT couch was divided by thickness into three sections for more accurate representation of attenuation along the length of the couch. Effects on surface dose are measured using a Tissue Maximum Ratio for the Varian EXACT couch, and Percent Depth Dose curves for the Varian IGRT and CIVCO extensions. Both CIVCO extensions were introduced into the Eclipse planning system by tracing the couch structures as in viewed in Ct images. All measurements were modeled in the Eclipse treatment planning system for comparison. Maximum attenuation was measured at 6MV with the smallest field for all couch setups. The EXACT couch produced a maximum attenuation of 19.2% and a surface dose increase of 47% of D[subscript lower case max] through the couch. Maximum attenuation for the IGRT couch and both CIVCO extensions were between 6-8%. Through the couch, the IGRT couch showed a surface dose increase of 29% of Dmax,; the CIVCO extensions produced a surface dose increase of 33% and 7% of Dmax. Maximum difference between measured and planned dose was 7%, for the EXACT couch, 1.9% for the IGRT couch and 2.4% for both CIVCO extensions. These treatment couches were shown to affect delivered and surface dose. Including the couch structure during treatment planning can account for the couch attenuation in most cases, but some attenuation values are underestimated in the planning software. Largest effects are for oblique treatment angles using low energy and small field sizes.