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Liquid mirrors promise to give us very large telescopes at low cost making them ideal instruments for cosmological studies. We review briefly the concept, past work, recent observations, and present unpublished optical shop tests. Tests on a 1.5-m liquid mirror indicate an excellent image and surface quality. Imagery and Ronchi tests indicate that we can obtain a point-spread function having a FWHM of the order of 0.3 arc second. In particular, bars having a width of 0.5 arc second and a separation of 1 arc second between their centers are clearly and cleanly resolved. Simple improvements should improve image quality even further. The mirror has a thin mercury layer that decreases costs and renders it less sensitive to vibrations and disturbances due to the wind. We discuss technological advances recently realized and under investigation in our laboratory. They promise to decrease costs to negligible levels, hopefully allowing us to construct very large telescopes dedicated to surveys and cosmology.