Wet process phosphoric acid (WPA) is produced by the attack of sulphuric acid on phosphate rock. The composition and purity of WPA depends mainly upon the composition of the rock used. In spite of the great variety of rocks, the different phosphoric acid processes and plant procedures, the WPA production problems are basically similar. These problems and their solutions are linked to the process stages: acidulation, filtration and concentration. Since rather low grade and quality rocks are increasingly processed world wide there is an upward trend to add mineral modifiers - clays and silicas, including a tailor-made perlite - to these rocks. They amend their behaviour and overcome problems in WPA production. They increase gypsum filterability and washing, reduce corrosion and fluorine emission and improve the quality of the concentrated acid product and derived fertilizers.