Ecosystem service flow is the key point and difficulty of ecosystem services research, which is now at the initial stage and show a developing trend of quantity and spatialization. However, the term "ecosystem service flow" is ambiguous. In this paper, we presented two descriptions about the term, and two corresponding theories and methods: the transmission of a service from ecosystems to people and distributed ecosystem model, and the flow of benefits accruing to people as generated by stocks of ecosystem structure and grey/black box theory. Then the latest researches related to the two descriptions were listed, and their merit and demerit were summarized. Finally, we pointed out several directions for future research in ecosystem service flow. Especially, the use of newest space technology and information technology (such as big data) and comprehensive understanding of ecosystem services (such as taking urban ecosystem services flow and abiotic flow into account) should be stressed in the future research.