The thioredoxin reductase (TrxR)-thioredoxin (Trx) system and the glutathione reductase (GR)-glutathione (GSH)-glutaredoxin (Grx) system are the two highly evolutionarily conserved "central nodes" of redox biology, which are of fundamental importance across all eukaryotes (Fig. ). They drive and buffer a range of biological redox reactions that are crucial to metabolism, protein folding, signaling, protein regulation, and many aspects of cellular homeostasis and stress responses. These systems are driven by reducing equivalents harvested from NADPH through the enzymes TrxR and GR, then distributed by downstream effector proteins, mainly isoenzymes of Trx and Grx, into various manifolds of dithiol/disulfidetype reactions. For all these redox enzymes, both chemocompatibility and protein-substrate binding determine their substrate scopes; these combine with subcellular compartmentalisation of the isoforms and substrates in each cascade to allow sophisticated regulation and spatial organisation of redox reactions in cells. Due to the fundamental importance of redox networks across biology, developing techniques to monitor and to respond to their dynamics is critical for understanding cellular physiology. Biological approaches to monitor redox biochemistry include redox-responsive ratiometric fluorescent protein fusions. These are well-established for imaging redox poise (the balance between reduced and oxidised fusion protein), and include sensors for Grxs and Trxs. However, they do not reveal the turnovers these species undergo, which are integral to a network understanding of redox homeostasis.