This thesis comprises two parts: the search for natural anti-inflammatory compounds and the identification of natural products that inhibit IDO. In silico strategies were applied to find new hit structures with potential anti-inflammatory activities. As a result of these screenings, a dichloromethane extract of Eriobotrya japonica which inhibited cPLA2 was discovered, whereas one fraction isolated fraction from Carthamus tinctorius was found to inhibit mPGES-1 and 5-LOX. Moreover, two new potent mPGES-1 inhibitors were indentified from Salvia officinalis: carnosol and carnosic acid (IC50: 5 M each), and in addition other compounds showing moderate inhibitory potency toward this enzyme. The major outcome of this research is the discovery of two new inhibitors of IDO, the lignans arctigenin and trachelogenin, showing IC50 values of 26.5 M and 56.4 M, respectively. Remarkably, matairesinol, a compound exhibiting close structural similarity with the two identified IDO inhibitors, showed very weak inhibitory activity. These results were the initial point of docking studies which revealed a possible binding mode of IDO inhibitors. In conclusion, by applying molecular modeling strategies in combination with ethnopharmacological analyses, several new anti-inflammatory and IDO-activities of plant extracts and single compounds of natural origin were discovered. These findings might contribute to the development of novel therapeutic agents for the treatment of inflammatory conditions, cancer, as well as other diseases.