Nitrosative stress, a nitric oxide (NO)-mediated nitrosylation of redox-sensitive thiols, has been linked to the regulation of signal transduction, gene expression, and cell growth and apoptosis and thus may be widely implicated in both physiological and pathological actions of NO. Protein S-nitrosylation has been observed to occur in vitro and in vivo in pathophysiological conditions. Apoptosis can be regulated by S-nitrosylation of the redox-sensitive cysteine residue in the active site of all caspase family proteases. Detection and measurement for the modification and inactivation of caspases by S-nitrosylation remain a new challenge because of the lability of the S-nitrosothiol moiety. This chapter describes approaches for assaying and identifying S-nitrosylated caspase enzymes in vitro and in vivo. These methods permit rapid and reproducible assays of S-nitrosylated caspases in biological and clinical specimens and should be useful for studies defining a pathophysiological role of NO in several apoptosis-associated human diseases.