The innermost regions of accreting BHs are a goldmine to understand how BHs work and influence their environments. During this talk I will focus on discussing some aspects related to a key property of actively accreting BH systems: their remarkable X-ray variability. In particular I will review results obtained from the study of the time-delayed response of the irradiated disc to the rapidly varying hard X-ray flux from the innermost Comptonizing region (X-ray reverberation). I will discuss applications of this analysis approach in both AGN and BHXRBs. These studies have been useful to map the geometrical structure of the inner accretion disc and the hard X-ray source, in some cases revealing complex morphological changes and providing us with key information to better understand the connection between the hard X-ray source and the jet.