Cellular senescence is a cell state implicated in various physiological processes and a wide spectrum of age-related diseases. Thus, accurate detection of senescent cells, especially in vivo, is essential especially since the field of senotherapeutics is growing rapidly. Here, we present a consensus from the International Cell Senescence Association (ICSA), defining and discussing key cellular and molecular features of senescence and offering recommendation on how to use them as biomarkers. We also present a resource tool to facilitate the identification of genes linked with senescence (SeneQuest, available at http://Senequest.net). Lastly, we propose an algorithm to accurately assess and quantify senescence, both in cultured cells and in vivo.