We discuss two methods for mid-infrared frequency comb generation in quadratic nonlinear materials. The first one of these methods is based on halfharmonic generation in a degenerate femtosecond optical parametric oscillator, which is synchronously pumped with a near-infrared laser frequency comb. The second method uses a continuous-wave-pumped optical parametric oscillator. In this approach, the quadratic nonlinearity is used not only for frequency down conversion, but also for the actual frequency comb generation from a continuouswave pump laser field. Both methods can be used to produce coherent broadband mid-infrared light that is ideal for real-time spectroscopic detection of small traces of biomarkers, toxics, and other interesting molecular compounds. In particular, these methods provide access to the 3-5 μm molecular fingerprint region, which contains strong fundamental vibrational stretching bands of hydrocarbons.