Publisher Summary This chapter describes seven-membered ring heteroatoms, with an emphasis on nitrogen, oxygen and sulfur. Systems of pharmacological interest or significance, and on future directions, are also discussed. While activity in this particular area has not been especially high, a number of interesting preparations and reactions are described in the chapter. A study of the rearrangement-ring expansion and rearrangement-ring opening reactions of stereoisomeric tetrahydropyrans with zinc acetate has been undertaken—for example, the oxepane 113 was obtained from 112 in moderate yield, together with a small amount of the ring opened product 114. The first total synthesis of the marine natural product (+)-rogioloxepane A containing a reduced oxepine ring has also been reported. New conditions for the Baeyer–Villiger oxidation continue to be explored including selenium-catalyzed oxidation with aqueous hydrogen peroxide and tin–zeolite as a chemoselective heterogeneous catalyst.