Accretion on to black holes is a key source of energy in the universe, but the regions closest to the black hole - where most of the potential energy is released - are inaccessible to imaging telescopes, at least into the distant future. Fortunately, nature has lent us a useful tool, in the form of multiwavelength continuum variability on time-scales of seconds to years, which carries much more information than spectra alone. I will show how we can interpret this variability to build a picture of the structure and behaviour of the accretion flow close to the black hole. I will first say what we think the variability is caused by, and show that accreting supermassive and stellar mass black holes have remarkably similar X-ray variability properties which scale in time with the black hole mass. I will then look at how X-ray and optical variability of AGN are correlated, giving us our first clear link from X-ray emitting corona to optical-emitting accretion disk. I will end by looking at how we can use X-ray reverberation to map the regions closest to the black hole, which will become a major industry with the next generation X-ray observatory, IXO, and is already just possible with existing instruments.

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