Mammals are the only vertebrates that have anucleate red blood cells (RBC’s). In this zoological class, RBC’s typically have the shape of biconcave discs. The cytomorphological study of RBCs in bloodstains is an issue with implications in fields such as forensic biology or prehistoric archaeology. Using scanning electron microscopy, the author has pioneered a new approach to the study of bloodstains, which has led moreover to a general terminology and systematics for smear-origin mammalian RBC’s. This paper summarises the results of more than 10 years of research in this field, by presenting the main morphological features of mammalian erythrocytes, when in smears.