Commonalities in Strong Line Emitters

Nebular emission lines are used to diagnose physical conditions such as temperatures, gas pressure, and abundances, as well as distinguishing radiation from star-formation and from black holes. Much of this comes from spectroscopy, but in order to identify emission line galaxies, one often needs narrow-band surveys or slitless spectroscopy. Without such techniques, many of these galaxies — which are often faint and low mass — would be missed or overlooked. I will discuss commonalities in the properties of emission line galaxies found at different redshifts.