We shall begin in a very traditional manner, with a discussion of the nomenclature of ethers. We will then describe how ethers may be prepared in the laboratory, and discuss the relative inertness of these compounds. A discussion of the chemistry of cyclic ethers follows, with particular emphasis on the preparation and reactions of epoxides—cyclic ethers containing a three‑membered ring. We will then introduce crown ethers—compounds that consist of large rings containing several oxygen atoms—and the spectroscopic properties of ethers. The unit will close with a description of the chemistry of thiols and sulfides, the sulfur‑containing analogues of alcohols and ethers.