Four persistent (Cygnus X-1, GX 339-4, GRS 1758-258 and 1E 1740.7-2942) and three transient (GS 2023+38, GRO J0422+32 and GS 1354-64) black hole X-ray binary systems have been extensively observed at radio wavelengths during extended periods in the Low/Hard X-ray state, which is characterised in X-rays by a hard power-law spectrum and strong variability. All seven systems show a persistent flat or inverted (in the sense that spectral index alpha >= 0) radio spectrum in this state, markedly different from the optically thin radio spectra exhibited by most X-ray transients within days of outburst. Furthermore, in none of the systems is a high-frequency cut-off to this spectral component detected, and there is evidence that it extends to near-infrared or optical regimes. Luminous persistent hard X-ray states in the black hole system GRS 1915+105 produce a comparable spectrum. This spectral component is considered to arise in synchrotron emission from a conical, partially self-absorbed jet, of the same genre as those originally considered for Active Galactic Nuclei. Whatever the physical origin of the Low/Hard X-ray states, these self-similar outflows are an ever-present feature. The power in the jet component is likely to be a significant (> 5%) and approximately fixed fraction of the total accretion luminosity. The correlation between hard X-ray and synchrotron emission in all the sources implies that the jets are intimately related to the Comptonisation process, and do not have very large bulk Lorentz factors, unless the hard X-ray emission is also beamed by the same factor.