Upconversion lasers represent a class of optically-pumped lasers that oscillate at frequencies higher than those used for pumping. Using nonlinear excitation processes, these lasers can convert infrared pump radiation from III-V semiconductor diode lasers, for example, to visible laser output. Thus upconversion pumping is an alternative to using parametric nonlinear optical processes such as frequency doubling for converting infrared laser radiation to coherent, short-wavelength output. The operation of a number of such upconversion lasers has been demonstrated in recent years using rare-earth ions doped into crystals and glasses as the laser medium. Upconversion lasers can potentially provide practical all-solid state sources with output at wavelengths from the ultraviolet to the red for a wide range of applications, including color displays, optical data storage, biomedical diagnostics, and underwater optical communication.