Optical frequency comb techniques are revolutionizing the art of measuring the frequency of light. The complex and highly specialized harmonic laser frequency chains of the past can now be replaced by a universal optical frequency comb synthesizer using just a single mode-locked femtosecond laser. After spectral broadening in a nonlinear optical fiber, such a laser emits a regular comb of hundred of thousands of evenly spaced sharp laser lines, extending throughout the visible and near infrared spectrum. The frequency of each line can be determined by radio frequency methods with extreme precision. Such frequency comb synthesizers are already revolutionizing precision laser spectroscopy, they provide the clock work for novel ultraprecise atomic clocks, based on optical transitions in atoms, ions or molecules, and they open new frontiers for stringent tests of fundamental physics laws.