Deep reinforcement learning (DRL) has been applied to a variety of problems during the past decade and has provided effective control strategies in high-dimensional and non-linear situations that are challenging to traditional methods. Flourishing applications now spread out into the field of fluid dynamics and specifically active flow control (AFC). In the community of AFC, the encouraging results obtained in two-dimensional and chaotic conditions have raised the interest to study increasingly complex flows. In this review, we first provide a general overview of the reinforcement-learning and DRL frameworks, as well as their recent advances. We then focus on the application of DRL to AFC, highlighting the current limitations of the DRL algorithms in this field, and suggesting some of the potential upcoming milestones to reach, as well as open questions that are likely to attract the attention of the fluid mechanics community.