The chapter deals mainly with the effect of impurities on the macro properties of metals made from steel, aluminium, copper, and zinc. It is about porosity, mechanical properties, strength, fatigue, and corrosion. It shows the effect of small amounts of inclusions and liquid impurities in bulk, on grain boundaries, and on the surface of the metals. The investigations have been carried out using mechanical tests, and advanced microscopic instruments like electron microscopes, X-ray diffraction, and different SIMS instruments (secondary ion mass spectrometry). The chapter lays the foundation for the need for refining of primary and secondary metals, i.e. recycling.