Three wave mixing was investigated with respect to its spectroscopic applications under fully resonant conditions. A theor. study of the appropriate 2nd order susceptibility c(2) reveals interesting features both for sum and difference frequency generation. For difference frequency generation, 2 different mechanisms are possible, 1 of which allows the observation of resonances between excited states and the study of dephasing induced coherent emission. The coherence decay rate between the 2 excited states may be measured and also its pure dephasing rate. The line-narrowing characteristics of the fully resonant c(2) were studied with respect to correlation between the inhomogeneous distribution for different levels. The line-narrowing capabilities are partly complementary for the sum-and the 2 difference frequency resonance configurations. This leads to the suggestion of new line-narrowing techniques which could provide information about type and amt. of correlation in inhomogeneously broadened systems.