In the last few years iron slag finds from the Late Avar Period (7-9th cent.), the Period of Hungarian conquest (10th cent.) and the Árpáds (11-13th cent.) uncovered in sites of excavation in the Carpathian Basin had been examined. The pieces of slags are by-products of the metallurgical process of bloomery and forming procedure of iron blooms. The structure of the examined slags was very heterogeneous from the metallographic point of view. The amorphous phase as well as panelled and dendritic crystallization can be found, which was the more frequent. Secondary, nay, tertiary branch of dendrite often can be studied, which was the result of gradual cooling. It was typical for its microstructure that minerals formed from oxides of elements with lower atomic number than iron (Al, Mg, Ca, etc.) surround the fayalite-rich parts. These minerals sometimes also have a high quantity of Fe, sometimes those have a very low Fe-content. The examination of microstructure can help to determine the type of the slag, whether it comes from metallurgical (as a tap-slag or cinder) or forming process, and to specify and reconstruct the metallurgical and physicochemical processes in the medieval bloomery.