Flexible Distribution Networks facilitates the enhanced flexibility, efficiency and reliability of distribution networks under complex operating environments delivered by high DG penetration. Considering the high share of DG and flexibility improvement brought by SOP-type power electronic devices, the work aims to solve the issues of optimal operation of flexible distribution networks. Technical solutions for flexible operation problems of FDN are discussed in depth, including efficient analytical methods with high penetration of DG, SOP-based optimal operation methods with uncertainty, and high-performance convex optimization algorithms. Readers will develop sound knowledge and gain insights into the optimal operation of FDN with high penetration of DG.