Redox signaling is a process whereby free radicals, reactive oxygen species, and other compounds act as biological messengers. A central mechanism underlying redox signaling is the oxidation of specific cysteine residues to modulate protein function. Oxidation of cysteine residues can change chemical properties, alter protein topography, and contribute to biological regulation. We seek to answer fundamental questions in the field, such as: Which redox-active components are present at which concentrations? Where in the cell are which redox conditions prevalent? Which cysteine residues in which proteins are modified under what conditions? How are biological processes influenced by thiol-based modifications?