Abstract: Black hole X-ray binaries (BHXRB) show complex, non-stationary behaviour during an outburst, characterized by significant changes of their spectral and timing properties. The origin of such behaviour is currently unknown, although evolution of the geometry of the innermost accretion flow is thought to play a major role. I will discuss open questions related to understanding the evolution of the inner structure of the accretion flow throughout an outburst, and present recent results obtained through the application of X-ray spectral-timing techniques to data from the latest generation of high throughput X-ray detectors.