  Black hole binary systems can emit very bright and rapidly varying X-ray
signals when material from the companion accretes onto the black hole,
liberating huge amounts of gravitational potential energy. Central to this
process of accretion is turbulence. In the propagating mass accretion rate
fluctuations model, turbulence is generated throughout the inner accretion
flow, causing fluctuations in the accretion rate. Fluctuations from the outer
regions propagate towards the black hole, modulating the fluctuations generated
in the inner regions. Here, I present the theoretical motivation behind this
picture before reviewing the array of statistical variability properties
observed in the light curves of black hole binaries that are naturally
explained by the model. I also discuss the remaining challenges for the model,
both in terms of comparison to data and in terms of including more
sophisticated theoretical considerations.
