Oxide cleanness today is one of the most important items of high-quality steel. But what is “Clean Steel” and how can it be produced? Macro inclusions produce defects in the product, micro inclusions usually are not harmful. A precondition for process technological developments are reproducible methods to measure the inclusion content. Because of the particle size distribution only heavy samples are prone to find large inclusions. Systems to avoid reoxidation and methods to improve separation of inclusions in tundish and mould are discussed. In this context the flow of steel plays an important role.