  Modeling emission lines from the millimeter to the UV and producing synthetic
spectra is crucial for a good understanding of observations, yet it is an art
filled with hazards. This is the proceedings of "Walking the Line", a 3-day
conference held in 2018 that brought together scientists working on different
aspects of emission line simulations, in order to share knowledge and discuss
the methodology. Emission lines across the spectrum from the millimeter to the
UV were discussed, with most of the focus on the interstellar medium, but also
some topics on the circumgalactic medium. The most important quality of a
useful model is a good synergy with observations and experiments. Challenges in
simulating line emission are identified, some of which are already being worked
upon, and others that must be addressed in the future for models to agree with
observations. Recent advances in several areas aiming at achieving that synergy
are summarized here, from micro-physical to galactic and circum-galactic scale.
