The aggregate light emitted by all extragalactic sources can be measured either as an absolute intensity or through its spatial fluctuations; these are known as line-intensity mapping (LIM) when a particular line transition is targeted. I will discuss how these measurements can be used both to learn about galaxy evolution and to investigate the presence of more speculative sources of radiation, such as decaying dark matter. I will also discuss the prospects of LIM as a probe of large-scale structure in a multi-tracer context, focusing on the joint information across different line transitions and across different tracers, such as galaxies and extragalactic CMB foregrounds.