The study of fast variability in X-ray binaries is one of the most powerful tools to study the accretion flow in the vicinity of compact objects. In particular, the comparison between power spectral features in black-hole and neutron-star binaries can provide important constrains for theoretical models. I review the main properties of the low-frequency quasi-periodic oscillations (LFQPOs) observed in black-hole candidates and show that they present clear analogies with those observed in neutron stars. I discuss and compare the main theoretical models in the light of this result.