To address the fundamental question of ‘Are we alone?’, a cornerstone of astrobiology, it is necessary to search for signatures of extraterrestrial life (biosignatures). This chapter is thus divided into two parts: in situ biosignatures and remote-sensing biosignatures. In the first, a variety of potential biomarkers are described, such as isotope ratios, individual and collective microfossils, homochirality (i.e., presence of molecules of the same handedness), distributions of biomolecular building blocks, and agnostic methods. In the second, the categories include gases (e.g., molecular oxygen and methane), surface components (e.g., pigments like chlorophylls), and temporal variations of certain features. This chapter concludes by delineating emerging criteria and techniques for evaluating the credibility of putative life detection.