Despite long and intensive investigation, the mechanisms by which nitric oxide (NO) regulates immune function and carcinogenesis remain incompletely understood. Protein S-nitrosylation, the covalent attachment of a nitroso group to a cysteine thiol, has emerged as a central mechanism of NO-dependent cellular regulation. In particular, recent research has revealed important roles for S-nitrosylation/denitrosylation in modulating the activity of macrophage and tumor cell proteins, implicating Snitrosylation in the regulation of macrophage function as well as in tumor development and response to therapy. This review summarizes recent progress in the identification and characterization of S-nitrosylated proteins in macrophages and cancer cells. The review highlights key findings and insights obtained from functional and proteomic studies about the roles of S-nitrosylation in signaling, transcription, apoptosis and other cellular processes relevant to macrophage function and cancer progression. Some of the implications of recent discoveries for the development of novel anticancer approaches are also discussed.