High-temperature confocal scanning laser microscopy (HT-CSLM) is a potent methodology for investigating various phenomena in the field of metallurgy. Initially applied to the observation of solid phase transformations and solidification, this method has gained traction in the field of non-metallic inclusion in steels in recent years. An overview of the experimental capabilities of HT-CSLM and the most important results of recent investigations regarding the topics of clean steel production are provided. It includes the formation of intragranular acicular ferrite (IAF) from the surface of non-metallic inclusions during the continuous cooling and heat treatment, which can be especially beneficial in the toughness of heat-affected zones of welded pieces. Furthermore, the investigation of agglomeration mechanisms of non-metallic inclusions (NMIs) in liquid steel is discussed to improve the insight into attraction forces between particles and clogging phenomena during continuous casting. Also, the dissolution of NMIs in various steelmaking slags can be observed by HT-CSLM to compare dissolution rates and mechanisms of NMI, where significant influences of temperature and chemical composition of the slag were shown. Last but not least, the experimental work regarding the interface between steel and slag is discussed, where novel techniques are currently being developed. A comprehensive summary of experimental techniques using HT-CSLM equipment to investigate different interactions of NMIs with steel and slag phases is compiled.