Quantum Efficiency (QE) is one of the most important parameters when either evaluating or using an imaging sensor for scientific applications. For back illuminated CCD and CMOS imagers, QE is determined by temperature, antireflection (AR) coatings, backside charging mechanisms, and silicon thickness. The accurate and precise measurement of QE requires careful consideration of illumination, temperature, calibration standards, optics, electronic equipment and components, and scattered light. QE is also closely related to the reflectance from the sensor surface. We present in this paper a study of the QE and reflectance from a variety of sensors used for astronomical imaging. Particular attention is given to precise calibration, temperature effects, models vs. measurements, and measurement techniques. We discuss all these issues and how they relate to the measurement and actual performance of sensors with different areas, thicknesses, and AR coatings.