Lens antennas are in widespread use in a variety of applications. Recently, an accurate method was proposed to design large-aperture, three-dimensional gradient-index (GRIN) slab lens antennas. Compared with traditional dielectric lenses of the same aperture, the proposed antennas promise superior performance. This is a typical application of metamaterials. This work is reported in SCIENCE CHINA Information Sciences, 2013, No. 12.