  The X-ray signal from active galactic nuclei and black hole X-ray binaries is
highly variable on a range of timescales. This variability can be exploited to
map the region of interest close to the black hole, which is far too small to
directly image for all but two black holes in the Universe. Spectral-timing
techniques provide causal information by combining timing and spectral
information. I present a brief review of such techniques, focusing on two
examples: X-ray reverberation mapping and phase-resolved spectroscopy of low
frequency quasi-periodic oscillations (LF QPOs). The former provides a means to
diagnose the accretion geometry and measure parameters such as black hole mass,
and the latter gives perhaps the best constraints we currently have as to the
enigmatic LF QPO mechanism.
