Fluoride is very important in making teeth more resistant to tooth decay.  This is not only for developing teeth, but also those which are fully erupted in the mouth.  The enamel of teeth is composed of calcium and phosphate minerals in a highly crystalline form called hydroxyapatite.  When fluoride is in the mouth it incorporates into the enamel and replaces the hydroxyapatite as fluorapatite.  This fluorapatite is stronger and is more resistant to an acid attack.  When fluoride is applied topically, such as in toothpastes or foam form, it can help to reverse early decay in a tooth by remineralization.  The fluoride adds minerals back into the damaged tooth surface to make it stronger and prevent the further progression of decay.