The Lyman alpha emission line has emerged as an important diagnostic tool, potentially useful to probe from the small scales of the interstellar medium out to the circumgalactic medium and even map the reionization history of the Universe. However, predicting the Lyman alpha emission (or lack thereof) from galaxies physical properties is still a challenge, implying that our ability to interpret high-redshift results is limited. Recently, the shape of the Lyman-alpha emission line profile has also been proposed as a way to pre-select Lyman Continuum (LyC) leaking galaxies. In this talk, I will present new results from a large sample of low-redshift galaxies with Lya emission observed at high spectral resolution with HST/COS. The spectra allow testing prevailing theories of Lyman alpha escape, including the relative role played by resonant scattering in outflowing gas and the geometry of neutral gas and dust.