Northern Corn Leaf Blight (NCLB) is an important leaf disease in maize worldwide. Setosphaeria turcica, causal agent of NCLB, infects and colonizes leaf tissue and induces elongated necrotic lesions. Disease control is based on fungicide application and resistant cultivars. Genetic resistance is mainly based on qualitative resistance. A race monitoring in Europe and South America indicated the presence of a distinct composition of races in different geographic regions. Moreover, histological studies were done to compare the pathogenesis of S. turcica races in maize exhibiting compatible and incompatible interactions. Maize plants from the reference line B37 with resistance genes Ht1, Ht2, Ht3 and Htn1 were inoculated with either incompatible or compatible races of S. turcica. Leaf segments from healthy and inoculated plants were collected one, three, six, twelve and eighteen days post inoculation (dpi) and stained with Chlorazol Black E. S. turcica formed appressoria and penetrated directly in both interactions. In a compatible situation, however, the pathogen was able to penetrate into xylem vessels at six dpi and to colonize the mesophyll at twelve dpi. In an incompatible interaction, the pathogen was not capable to colonize neither the xylem vessels nor the mesophyll. Further studies are underway in order to better understand the resistance mechanisms involved in incompatible interactions and the individual functions of the different known resistance genes.