Significance: The oxidative process of disulfide-bond formation is essential for the folding of most secretory and membrane proteins in the endoplasmic reticulum (ER). It is driven by electron relay pathways that transfer two electrons derived from the fusion of two adjacent cysteinyl side chains onto various types of chemical oxidants. The conserved, ER-resident endoplasmic oxidoreductin 1 (Ero1) sulfhydryl oxidases that reduce molecular oxygen to generate an active-site disulfide represent one of these pathways. In mammals, two family members exist, Ero1a and Ero1b. Recent Advances: The two mammalian Ero1 enzymes differ in transcriptional and posttranslational regulation, tissue distribution, and catalytic turnover. A specific protein–protein interaction between either isoform and protein disulfide isomerase (PDI) facilitates the propagation of disulfides from Ero1 via PDI to nascent polypeptides, and inbuilt oxidative shutdown mechanisms in Ero1a and Ero1b prevent excessive oxidation of PDI. Critical Issues: Besides disulfide-bond generation, Ero1a also regulates calcium release from the ER and the secretion of disulfide-linked oligomers through its reversible association with the chaperone ERp44. This review explores the functional repertoire and possible redundancy of mammalian Ero1 enzymes. Future Directions: Systematic analyses of different knockout mouse models will be the most promising strategy to shed new light on unique and tissue-specific roles of Ero1a and Ero1b. Moreover, in-depth characterization of the known physical interactions of Ero1 with peroxidases and PDI family members will help broaden our functional and mechanistic understanding of Ero1 enzymes.