  Since radio continuum observations are not affected by dust obscuration, they
are of immense potential diagnostic power as cosmological probes and for
studying galaxy formation and evolution out to high redshifts. However, the
power-law nature of radio frequency spectra ensures that ancillary
spectroscopic information remains critical for studying the properties of the
faint radio sources being detected in rapidly-increasing numbers on the pathway
to the Square Kilometre Array. In this contribution, I present some of the key
scientific motivations for exploiting the immense synergies between radio
continuum observations and multi-object spectroscopic surveys. I review some of
the ongoing efforts to obtain the spectra necessary to harness the huge numbers
of star-forming galaxies and AGN that current and future radio surveys will
detect. I also touch on the WEAVE-LOFAR survey, which will use the WEAVE
spectrograph currently being built for the William Herschel Telescope to target
hundreds of thousands of low frequency sources selected from the LOFAR
continuum surveys.
