The spectrum of a frequency comb, commonly generated by a mode-locked femtosecond laser, consists of several hundred thousand precisely evenly spaced spectral lines. Such laser frequency combs have revolutionized the art of measuring the frequency of light, and they provide the long-missing clockwork for optical atomic clocks. As tools for precision spectroscopy, notably of atomic hydrogen, laser combs permit stringent tests of fundamental physics laws. New applications are evolving which go far beyond the original purpose. As calibration tools in astronomy, frequency combs are facilitating the search for exoplanets, and they may lead to direct evidence for the accelerating expansion of space in our universe. Laser combs are also becoming powerful instruments for broadband molecular spectroscopy. They can dramatically improve the resolution and recording speed of Fourier spectrometers, and they are creating intriguing new opportunities for highly multiplexed nonlinear spectroscopy and microscopy.