High penetration of renewable energy sources (RESs) imposes several techno-economic challenges to distribution system operators (DSOs) due to their variability in power generation and, hence, increases the need for additional operational flexibility. Operational flexibility aims at securely covering the possible variations at the minimum cost using emerging flexible alternatives or designing novel local market mechanisms to incentivize flexibility providers. In such a situation, the DSOs can use the potential of flexible options such as energy storages (ESs), demand response (DR), plug-in electric vehicles (PEVs), or on-site fast run generators. However, each of the mentioned flexible resources has its own specific characteristics and requirements that should be taken into account, and this raises the complexity. Optimal network reconfiguration schemes are the other solution for increasing power system flexibility at the distribution level. There is a great research gap related to renewable-based distribution network planning from a flexibility point of view. Therefore, this book aims to discuss the additional flexibility needs introduced by RESs and describe general approaches to analyze the need for and provision of additional flexibility in future distribution networks at both the planning and operational time frames. This book successfully suggests new solutions and techniques to increase the flexibility in distribution systems. It also highlights the needs for moving towards smart distribution grids in order to enhance the flexibility in modern and future power systems.