We describe the optical design and performance of ’QUEST and DASI’ or ’QUaD’, a ground-based high-resolution experiment designed to measure the polarisation properties of the cosmic microwave background radiation. QUaD uses bolometric detectors at 100 and 150 GHz on a 2.6 m Cassegrain telescope. The QUaD optics are designed to minimise systematic effects as well as to maximise sensitivity, and we report here on the comprehensive quasi-optical analysis used to achieve this design. We also present initial optical performance measurements achieved in operation, and discuss changes made to the optics to overcome some errors in the mechanical construction of the primary mirror. The QUaD experiment is now fully operational and taking world-leading data at the South Pole.