High-speed steels (HSS) are high-alloyed ledeburitic tool steels that are mainly used for cutting applications. The characteristic properties of HSS such as high wear resistance, high (hot) hardness and temper resistance are predominantly attained by the combination of the alloying elements tungsten, molybdenum and vanadium. However, the unfavorable price trend of these elements in the last few years led to a change of alloying concepts. Consequently, new aluminium-alloyed HSS were developed. Aluminium is not a typical alloying element in HSS. The influence of aluminium was investigated during the Second World War at the Montanuniversität Leoben for the first time. At this time there was an enormous consumption or rather a shortage of molybdenum and especially of tungsten worldwide. Tungsten was not only used for the HSS production, but was also extremely needed for the war industry. Attempts were made in order to replace these alloying elements in HSS with cheaper elements. It was found out that a certain percentage of tungsten and molybdenum in HSS can be replaced or substituted by an aluminium content of 0.3 up to 1.4 mass-% without any loss of cutting performance. However, no detailed metallurgical investigations were carried out to explain and clarify this behaviour of aluminium. The influence of aluminium in HSS is highly controversial due to the fact that aluminium compared to tungsten and molybdenum is not a carbide-forming element but the special properties of HSS are mainly gained by the presence of carbides. Despite several works in this field there is no clear understanding how aluminium can affect the properties of HSS. In the present work investigations were conducted on three HSS grades with varying aluminium contents only. Metallurgical analyses by means of standard and high sophisticated examination methods are carried out to investigate and clarify the influence of aluminium in high-speed steels.