Polyurethanes are normally prepared by reacting a hydroxyl compound with a di or polyisocyanate. Isocyanates are much more versatile in their reaction permitting many other links to form. In polyurethane applications also urea-linkages,  isocyanurate, amide and carbodiimide linkages can be formed. The high Tg of the urea linkage and the high heat resistance of the isocyanurate linkage make these linkage of considerable importance in polyurethanes. Every end use and every application requires a different polyol and isocyanate.  This is an overview of the most common applications of polyurethanes and the composition of these systems. Because of the many different applications and end-uses there are many other modifications in polyols and isocyanates and other additives in use.