This lecture will describe how regulated amino acid metabolism controls ferroptosis in a cell-extrinsic way. The focus will be on tryptophan metabolism by two oxidases, IL4i1 (a secreted protein) and IDO1 (cytoplasmic). Both enzymes are expressed in myeloid cells infiltrating the tumor microenvironment. Biochemically, both IL4i1 and IDO1 generate AhR ligands in addition to depleting tryptophan. However, the main discoveries described here will center on the metabolites that suppress ferroptosis in neighboring cells. In addition to the cellular biochemistry of this system, implications for cancer biology will be covered.