This chapter introduces the biology of amphioxus and discusses the research history of this living fossil. Amphioxus was first discovered in Europe as a lancelet. Taxonomically, it belongs to the subphylum Cephalochordata, which means "chordate in the head." The fossil evidence, as well as the modern molecular phylogenetic tree, suggests amphioxus as the most basal chordate. The body plan and embryonic development of amphioxus are close to its vertebrate ancestors. It was used as a model animal for developmental studies since the nineteenth century. The famous Chinese zoologist Tung Tichou made many contributions to the developmental research of amphioxus. Amphioxus can now be used as a model for the study of the origin of vertebrate immunity. © 2016 China Science Publishing & Media Ltd. Published by Elsevier Inc.