| Note that this colloquium announcement is one day early. We need tomorrow's AJDI slot for the retirement of Sjouke Zwier and Hans Tenkink
The variable X-ray emission from accreting black holes encodes information about the structure and short-term evolution of the emitting regions closest to the black hole, but for many years it has been difficult to interpret these variations. Recent years have seen rapid developments in our understanding of the variability and what it tells us about the central regions, driven by a better understanding of how to use the combined energy and time information from X-ray data (so called 'spectral-timing') to understand the causal connections between the different emitting components. I will give a brief review of these exciting developments and a look forward to future advances with new X-ray missions.