High-performance mirrored optics are an enabling technology in a variety of high-sensitivity measurement techniques such as cavity-based absorption spectroscopy, optical atomic-clock spectroscopy, ring-laser gyroscopes (RLGs), and gravitational-wave detection. In each technique, high-reflectivity, low-loss mirrors that can operate under a variety of environmental conditions without any sacrifice in spectral performance are critical. Here, we describe how mirrors manufactured by ion beam sputtering (IBS) and using dielectric materials are designed to achieve extremely high reflectivity (R) greater than 99.99% and low loss over a broad spectral range. Particular emphasis is given to how the losses associated with light absorption, scattering, and transmission are minimized and quantified by direct measurement. An example of the importance of these mirrors in cavity ring-down spectroscopy (CRDS) will also be discussed.