Femtosecond mode-locked lasers producing visible/infrared frequency combs have steadily advanced our understanding of fundamental processes in nature. For example, optical clocks employ frequency-comb techniques for the most precise measurements of time, permitting the search for minuscule drifts of natural constants. Furthermore, the generation of extreme-ultraviolet attosecond bursts synchronized to the electric field of visible/infrared femtosecond pulses affords real-time measurements of electron dynamics in matter. Cavity-enhanced high-order harmonic generation sources uniquely combine broadband vacuum- and extreme-ultraviolet spectral coverage with multimegahertz pulse repetition rates and coherence properties akin to those of frequency combs. Here we review the coming of age of this technology and its recent applications and prospects, including precision frequency-comb spectroscopy of electronic and potentially nuclear transitions, and low-space-charge attosecond-temporal-resolution photoelectron spectroscopy with nearly 100% temporal detection duty cycle.