The process of oxidative protein folding in the endoplasmic reticulum (ER) generates hydrogen peroxide, a reactive oxygen species (ROS). The secretory load of a cell can vary widely, and cells require mechanisms to maintain folding fidelity within the ER despite fluctuating ROS levels. One such mechanism involves tuning the activity of the ER Hsp70 chaperone, BiP. Our lab established that a conserved cysteine residue within the nucleotide-binding domain of S. cerevisiae BiP (Kar2) is oxidized in response to ROS accumulation, and oxidation causes the chaperone to bind peptides with perpetually high affinity. The modified peptide-binding activity of oxidized BiP imparts a protective advantage to cells during oxidative stress by limiting aggregation of proteins damaged by ROS. Although beneficial when ROS levels are high, perpetual BiP modification and the holding of peptides decreases cell fitness under non-stressed conditions. Thus, it is essential that cells coordinate BiP oxidation to maintain folding homeostasis with the changing ER redox status. Here we uncover roles for the BiP nucleotide exchange factor, Sil1, and the oxidative protein folding enzyme, protein disulfide isomerase (PDI), in controlling the redox state of BiP In Chapter 2, we show that Sil1 possesses a N-terminal cysteine pair capable of reducing the redox-sensitive BiP cysteine. The unexpected reductant activity of Sil1 enables BiP to resume its non-stressed peptide binding activities. In Chapter 3, we identify PDI as a further regulator of BiP oxidation as PDI reduces BiP both directly and by providing electrons to activate Sil1. Due to its role in oxidative protein folding, the redox state of PDI (and its capacity to give electrons to BiP) is tightly linked to the current redox status of the ER. We suggest that together PDI and Sil1 serve as sensors of the ER redox state that adjust BiP function to match the ER redox status.