HIILines analytically models lines emitted by the ionized interstellar medium (ISM). It covers [OIII], [OII], H_α, and H_β lines. The strength of HIILines is its high computational efficiency. It can be used for galaxy spectroscopic survey measurement interpolations assuming a one-zone picture and galaxy line emission measurement design and forecasts. HIILines also performs post-processing of hydrodynamical galaxy formation simulations for ISM emission lines.