Total oxygen analysis, chemical or electrolytic extraction and optical microscope with image analyzer are common methods to measure the cleanliness of steel. A re-melting technique was developed to float out inclusions and be examined under SEM or Cathodoluminescence microscopy. Evaluation of inclusions by optical emission spectrometer, ultrasonic tests after hot rolling, slab milling and defect mapping as well as a computer-controlled SEM analysis are described. As a result, the source causes of defects may be identified and countermeasures developed. Tundish nozzle clogging is quantified by a nozzle clogging factor, which is monitored on-line. Inclusion characterization and the implementation of a clean steel practice have led to a reduction of nozzle clogging and an increased cast string length with improved steel quality. Total oxygen analysis, chemical or electrolytic extraction and optical microscope with image analyzer are common methods to measure the cleanliness of steel. A re-melting technique was developed to float out inclusions and be examined under SEM or Cathodoluminescence microscopy. Evaluation of inclusions by optical emission spectrometer, ultrasonic tests after hot rolling, slab milling and defect mapping as well as a computer-controlled SEM analysis are described. As a result, the source causes of defects may be identified and designed measures developed. Tundish nozzle clogging is quantified by a nozzle clogging factor, which is monitored on-line. Inclusion characterization and the implementation of a clean steel practice have led to a reduction of nozzle clogging and an increased cast string length with improved steel quality.