The main goal of this project was to design and test a set of collimators for use in measuring x-ray attenuation. Two collimators: a primary collimator and a secondary collimator, were built. The primary collimator was built using 10.0 cm of lead, followed by 7.0 cm of tin, 1.5 mm of copper and 0.5 mm of aluminum, in the respective order. A 1.5 mm thick, copper filter was also used on the aperture of the primary collimator. The secondary collimator was built using 2.5 cm of lead. The cross-sectional size of both the collimators was made to be 15 x 15 cm2. The shape of the aperture in each collimator was made to be 1.5 cm x 3.0 cm. Various features of the collimators were tested in the Varian TrueBeam and Varian Trilogy linear accelerators. The primary collimator was found to best minimize the secondaries produced by the various components inside the gantry and the collimator itself that reach the absorber when placed at least 30.0 cm away from the absorber upstream the beam. The copper filter in the primary collimator was found unnecessary. The secondary collimator was found to best minimize the secondaries produced by the absorber that reach the detector, when placedat least 20.0 cm away from the detector upstream the beam. With both the collimators placed at their optimal positions, the cross-sectional size of the collimated beam measured around the detector, placed at 200 cm from the source, was found to be approximately equal to 1.5 cm x 3.0 cm. The cross-sectional size of the collimated beam at the position of absorber was found to be 1.9 cm x 4.3 cm and that of attenuated x-ray beam was found to be 3.0 x 5.5 cm.