Neutral diffuse intergalactic gas that existed during the Epoch of Reionization ( EoR ) suppresses Ly \alpha flux emitted by background galaxies .
In this chapter I summarise the increasing observational support for the claim that Ly \alpha photons emitted by galaxies at z > 6 are suppressed by intervening HI gas .
I describe key physical processes that affect Ly \alpha transfer during the EoR .
I argue that in spite of the uncertainties associated with this complex multiscale problem , the data on Ly \alpha emitting galaxies at z = 0 - 6 strongly suggests that the observed reduction in Ly \alpha 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 .
Current models favor a volume filling factor of neutral gas of x _ { HI } > 0.4 at z~ { } 7 .
I summarise how future surveys on existing and incoming instruments are expected to reduce existing observational uncertainties enormously .
With these improved data we will likely be able to nail down reionization with Ly \alpha emitting galaxies .