Fatty acids are monocarbonic acids with chain lengths between 2 and 26 (and sometimes more). Most fatty acids in plants and animals have an even carbon number and no branching. One or more double bonds may occur, mostly in cis configuration. More than 40 different fatty acids are commonly encountered in foods. This chapter goes into extensive depth on fatty acids. Unhealthful expansion of fat stores due to persistent overfeeding is fast becoming the norm in the United States and many other affluent countries. The major killers of older adults (type 2 diabetes and cardiovascular disease) have held their ground despite large advances in medical technology, and now they reach into adolescent and even childhood populations. Overfeeding is discussed in depth in this chapter. The latter half of this chapter is devoted to the metabolism of acetate (acetic acid), myristate (myristic acid), conjugated linoleic acid (CLA), docosahexaenoic acid (DHA), trans-unsaturated fatty acids, and chlorophyll.