Systems and methods for high resolution and high sensitivity spectroscopy are disclosed. High resolution can be obtained in conjunction with comb sources via comb resolved spectroscopy. For example, Fourier transform spectroscopy with a scan range larger than a cavity round trip time of the comb sources can be used to obtain comb resolution, where it may be useful to match the comb lines of the source with the sampling points of the Fourier transform spectrometer. High sensitivity can be obtained using multiple passes through a gas cell, cavity enhanced spectroscopy, cavity ring-down spectroscopy, or photo-acoustic spectroscopy. Fiber or solid-state lasers as well as semiconductor or quantum cascade based lasers can be used as comb injection sources. These sources can also be combined with nonlinear frequency broadening techniques via supercontinuum generation, DFG, OPOs or OPAs.