The main task of this diploma thesis was to gain knowledge about the behaviour of nonmetallic inclusions in electroslag- refining. For this purpose, a chosen plastic mould steel, BÖHLER M333, was remelted with different slag types to detect possible influences on those inclusions. The main idea of the literature study was to summarize all refining- mechanism of the inclusions during the remelting process. Using the inclusion-analyser (PSEM), one electrode sample and one further block sample in each test series were investigated. The gained knowledge shows that the type of slag actually do have a strong influence on the resulting inclusions in a block. An inclusions identification of the block samples with the scanning electron microscope confirmed the PSEM- results. Using metallographic investigations on the electrode tips, values for the dimensions of the liquid metal film and the heat affect zones were attained. The relationship of those gained values with the degree of the resolution of the inclusions was also shown. Thermodynamic resolution calculations have been carried out, additionally, for the dominating inclusion types in the liquid steel matrix on the electrode tips to show the resolution times, which are strongly dependent on the inclusion type.