Electric vehicle massive integration into power systems will pose significant challenges, in particular to distribution grids. However, they can be a source of flexibility for the power grid operation and planning. In this work, we systematically identified the key technical and economic aspects for the proactive integration of EVs into electricity grids as providers of flexibility services. Technical aspects have been addressed in literature, analysing EV impacts and smart charging strategies at various levels of the power systems. Main barriers come from economic and institutional aspects, such as the absence of frameworks for local flexibility trading, uncertain viable business models and the evolution of roles and responsibilities of DSOs as well as their interactions with other stakeholders. However, there have been significant advances in recent years, with demonstrator projects proposing new technical solutions and testing business models, increasing interest in exploiting flexibility at the distribution level from EU regulators, and the development of local flexibility mechanisms by DSOs.