Abiotic stresses often cause metabolic imbalances which affect cellular redox homeostasis and alter the rate of reduction state of functional and regulatory protein thiols and the rate of reactive oxygen species release. Excessive displacement from redox equilibrium causes oxidative damage to cell structures and may elicit cell death. The understanding of the cell response to progressive stress must include knowledge on the thiol redox state of specific proteins. This chapter describes selected gel-based biochemical methods (i) to identify thiol–disulfide redox proteins that undergo major redox-dependent conformational changes by 2D redox SDS-PAGE and (ii) to determine the thiol redox state of proteins by sequential blocking and labeling with N -ethylmaleimide and methoxypolyethylene glycol maleimide-5000 (mPEG-Mal-5000). Both sets of methods provide experimental information that defines the redox proteome of the cell.