  Galaxy surveys targeting emission lines are characterising the evolution of
star-forming galaxies, but there is still little theoretical progress in
modelling their physical properties. We predict nebular emission from
star-forming galaxies within a cosmological galaxy formation model. Emission
lines are computed by combining the semi-analytical model \sag\ with the
photoionisation code \mapp. We characterise the interstellar medium (ISM) of
galaxies by relating the ionisation parameter of gas in galaxies to their cold
gas metallicity, obtaining a reasonable agreement with the observed \ha, \oii,
\oiii\ luminosity functions, and the the BPT diagram for local star-forming
galaxies. The average ionisation parameter is found to increase towards low
star-formation rates and high redshifts, consistent with recent observational
results. The predicted link between different emission lines and their
associated star-formation rates is studied by presenting scaling relations to
relate them. Our model predicts that emission line galaxies have modest
clustering bias, and thus reside in dark matter haloes of masses below $M_{\rm
halo} \lesssim 10^{12} {[\rm h^{-1} M_{\odot}]}$. Finally, we exploit our
modelling technique to predict galaxy number counts up to $z\sim 10$ by
targeting far-infrared (FIR) emission lines detectable with submillimetre
facilities
