This chapter provides an update of the chemistry of heterocycles based on the naphthalene core with two heteroatoms in each ring but no bridgehead, excluding the pteridines, up to August 2006. The chemistry of the diazinodiazines was reviewed in CHEC(1984), Chapter 2.17, and this was expanded in scope and updated in CHEC-II(1996), Chapter 7.19. Since the appearance of CHEC-II(1996), a great deal of new chemistry of the diazinodiazines has been published, with particular advances in the chemistry of the pyridazinopyrimidines and pyrimidopyrimidines. Notable advances have also been reported in the chemistry of the diazinooxazines and, to a lesser extent, the diazinothiazines. Much of the information included in this update comes from the patent literature, and is therefore not readily available through other journal sources.