In the run up to routine observations with the upcoming generation of radio facilities, the nature of sub-mJy radio population has been hotly debated. In this talk, we describe multi-frequency data designed to probe the emission mechanism that dominates in these faint radio sources. Our analysis is based on observations of the Lockman Hole using the Giant Metre-wave Radio Telescope (GMRT) -- the deepest 610-MHz imaging yet reported -- together with 1.4-GHz imaging from the Very Large Array (VLA), well matched in resolution and sensitivity to the GMRT data (rms$_{610MHz}\sim 15 \mu$Jy/beam, rms${_1.4GHz}\sim 6\mu$Jy/beam, FWHM$\sim$5 arcsec). The GMRT and VLA data are cross-matched to obtain the radio spectral indices for the faint radio emitters. Statistical analyses show no clear evolution for the median spectral index, alpha(1.4GHz, 610MHz) (where $S_{\nu} \propto \nu^{\alpha}$), which is found to be approximately -0.6 to -0.7 based on an almost unbiased 10-sigma criterion, down to a flux level of S$_{1.4GHz}\geq 100\mu$Jy. The fraction of inverted spectrum sources, $\alpha$(1.4GHz, 610MHz) $\geq$ 0, is less than 10 per cent. The results suggest that the most prevalent emission mechanism in the sub-mJy regime is optically-thin synchrotron, ruling out a dominant flat spectrum or ultra-steep spectrum radio population. The spectral index distribution has a significant scatter, Delta($\alpha$) $\sim$ 0.4-0.5, suggesting a mixture of different populations at all flux levels. This is supported by spectroscopic classifications of radio sources with hard X-ray emission, which has allowed us to estimate that the fraction of radio-quiet (type 1 and 2) AGN at S$_{1.4GHz} \leq 300\mu$Jy is 13--30 per cent, in agreement with a dominant star-forming galaxy population in the sub-mJy regime.