The literature on how Mg is present in tooth enamel and on the incorporation of Mg in synthetic apatites is reviewed. Then a theoretical consideration is given on the basis of ionic radii about the incorporation of Mg in F-, Cl- and OH-apatite. Finally, the results are given of experiments on the incorporation of Mg in fluor, chlor, hydroxy, and carbonate apatite. It appears that the extent of incorporation of Mg in the apatite lattice, if at all, is very limited and that Mg in tooth enamel is to a very limited extent incorporated in the apatite lattice, the main portion being surface bound or present in a separate phase.