Abstract: Highly volatile distributed generators（DGs）,large scale electric vehicles and various controllable demand-side resources have increasingly been integrated into smart distribution networks.It puts forward a higher requirement for the flexibility of distribution networks to cope with the various uncertainties.Characteristics of the flexibility are analyzed from the point of view of the evolution of future distribution networks.The demand and multi-dimensional characteristics of distribution network flexibility are elaborated.The key technologies,including the observability,controllability and quantitative analysis of the flexibility,are discussed.Finally,apreview is performed from the perspective of flexibility for the future development of distribution network planning,operating and control technologies.