During recent years degenerate four-wave mixing (DFWM) has proven to be a powerful spectroscopic tool for the determination of major and minor species in jets, cells, flames and discharges under a large variety of pressures. The wavelengths used cover a range from the ultraviolet to the mid infra-red. In contrast to laser induced fluorescence (LIF) DFWM shows some completely different features: It is a four-wave mixing process, generating a laser-like signal beam being phase conjugated to the socalled probe beam. This is possible because of a nonlinear response of the medium being subject to strong laser radiation. Since this signal beam can be nearly background-free detected, which is not the case using LIF (characterized by an isotropic emission of radiation), the method has some general advantages compared to LIF. The resolution of molecular transitions is remarkably better compared to LIF, the unwanted problems related to quenching effects at higher pressures known from LIF are also rather low. Last but not least DFWM can - in principle - provide a one-shot tomography using only three laser beams penetrating the medium and one outcoming to be detected. For these reasons DFWM is currently applied and tested in view of future combustion flame diagnostics of Diesel and Otto motors with special attention to the determination of NO-densities and temperatures.