  Neutral diffuse intergalactic gas that existed during the Epoch of
Reionization (EoR) suppresses Lyman Alpha (Lya) flux emitted by background
galaxies. In this chapter I summarise the increasing observational support for
the claim that Lya photons emitted by galaxies at z>6 are suppressed by
intervening HI gas. I describe key physical processes that affect Lya transfer
during the EoR. I argue that in spite of the uncertainties associated with this
complex multiscale problem, the data on Lya emitting galaxies at z=0-6 strongly
suggests that the observed reduction in Lya flux from galaxies at z>6 is due to
additional intervening HI gas. The main question is what fraction of this
additional HI gas is in the diffuse neutral IGM. I summarise how future surveys
on existing and upcoming instruments are expected to reduce existing
observational uncertainties enormously. With these improved data we will likely
be able to nail down reionization with Lya emitting galaxies.
