What is the geometry of the accretion flow around accreting black holes (BH), and how does it evolve with the accretion state of the source? X-ray reverberation measurements are shedding some light on these outstanding questions. By measuring time delays associated with the reprocessed signal from gas orbiting the BH this technique allows us to map the innermost regions of accreting-BH systems. The recent results and future prospects in this field will also be discussed.