Biomarkers are increasingly being used in environmental monitoring to provide evidence that organisms have been exposed to, or affected by, xenobiotic chemicals. Usually, these biomarkers rely on biochemical, histological, morphological, and physiological changes in whole organisms; however, changes at the cellular and molecular levels of organization, especially in nucleic acids and proteins, are increasingly being used to supplement these more traditional biomarkers. This chapter starts by giving a brief overview of biomarkers and some of the basic requirements for their effective use. Then stress-inducible proteins that are potentially useful as environmental biomarkers are explored, and some examples of their application as biomarkers and methods of detecting them are presented.