This chapter summarizes some of the main insights into the concept of flexibility provision from distributed energy resources (DER) and relevant grid and market services that DER could provide. More specifically, after giving a general view of flexibility drivers and requirements in low-carbon power systems, a general definition and useful characterization of the concept of DER flexibility and their deployment for provision of grid and market services are provided. In order to model, characterize, and assess DER flexibility, particularly in the context of DER aggregation and considering local network constraints, the concept of nodal operating envelope is introduced with the use of active power-reactive power maps to represent the upstream services that DER aggregation could provide while maintaining network safety and reliability. Information is then complied from selected DER technologies about their potential to provide flexibility over different time scales and in different energy forms, e.g., renewable generation (e.g., inverter-interfaced wind), energy storage (e.g., batteries, electric vehicles, thermal storage, and seasonal storage), and demand response (e.g., buildings and district energy systems), and to illustrate the role of network technologies in facilitating DER flexibility. The interactions among different stakeholders and different aggregation options (e.g., virtual power plants, microgrids, community and district energy systems, distributed markets, etc.) in the context of technical and commercial provision of DER flexibility services are then analyzed. Finally, general regulatory and market aspects that could facilitate full integration of DER flexibility into the electricity value chain are discussed.