  Relativistic outflows are a common phenomenon in accreting black holes.
Despite the enormous differences in scale, stellar-mass black holes in binaries
and supermassive black holes in Galactic Nuclei produce jets with analogous
properties. In both are observed two types of relativistic outflows: 1) steady
compact jets with flat-spectrum, and 2) sporadic extended jets with
steep-spectrum and apparent superluminal motions. Besides, the most common
class of gamma-ray bursts are afterglows from ultra-relativistic jets
associated to the formation of black holes at cosmological distances.
