Biomarkers, also known as biological markers, are characteristic chemicals, chemical complexes, biominerals, accumulations of trace elements, fractionations of stable isotopes, spectral features, and other indicators that provide clues to the presence of extant or past biological activity. Establishing what are reliable biomarkers, in extraterrestrial rocks or other samples and in the spectra of exoplanets, and thereby developing a catalog of biosignatures, is crucial in the search for life beyond the Earth.