"The advent of laser frequency combs a decade ago has revolutionized optical frequency metrology. Such combs have become enabling tools for a growing tree of applications, from optical atomic clocks to attosecond science.Recently, the millions of precisely controlled laser comb lines produced with a train of ultrashort laser pulses have been harnessed for highly multiplexed molecular spectroscopy.Fourier multi-heterodyne spectroscopy with frequency combs is emerging as a powerful new spectroscopic tool. Cavity-enhanced absorption spectroscopy with two frequencycombs just demonstrated a dramatically improved sensitivity, compared to conventional Fourier spectroscopy, with recording times shortened from seconds to microseconds.Such capabilities open exciting opportunities for instantaneous trace gas analysis, time-resolved spectroscopy of short-lived molecular species, precision spectroscopy andhyperspectral imaging. Moreover, since frequency combs involve intense ultrashort laser pulses, nonlinear interactions can be harnessed, such as saturation or coherent transientphenomena including photon echoes, in analogy to multi-dimensional NMR spectroscopy. Envisioned applications range from optical labeling for the simplification anddisentanglement of complex spectra to coherent control for the selective microscopic imaging of unlabeled biomolecules.Such new spectroscopic methods will be initially explored with state-of-the-art frequency comb sources, based on femtosecond fiber lasers and nonlinear conversion. Novel compact and reliable spectroscopic instruments with unprecedented capabilities will become possible with frequency comb generators based on cascaded four wave mixing in toroidal micro-resonators."