Observations of high-redshift Ly- \alpha sources are a major tool for studying the high-redshift Universe and are one of the most promising ways to constrain the later stages of reionization .
The understanding and interpretation of the data is far from straightforward , however .
We discuss the effect of the reionizing intergalactic medium on the observability of Ly- \alpha sources based on large simulations of early structure formation with radiative transfer .
This takes into account self-consistently the reionization history , density , velocity and ionization structures and nonlinear source clustering .
We find that all fields are highly anisotropic and as a consequence there are very large variations in opacity among the different lines-of-sight .
The velocity effects , from both infall and source peculiar velocity are most important for the luminous sources , affecting the line profile and depressing the bright end of the luminosity function .
The line profiles are generally asymmetric and the line centers of the luminous sources are always absorbed due to the high density of the local IGM .
For both luminous and average sources the damping wing effects are of similar magnitude and remain significant until fairly late , when the IGM is ionized between 30 % and 70 % by mass .

The ionizing flux in the ionized patch surrounding a high density peak is generally strongly dominated , particularly at late times , by the cluster of faint sources , rather than the central massive galaxy .
Our results reproduce well the observed mean opacity of the IGM at z \sim 6 .
The IGM absorption does not change appreciably the correlation function of sources at high redshift .
Our derived luminosity function assuming constant mass-to-light ratio provides an excellent match to the shape of the observed luminosity function at z = 6.6 with faint-end slope of \alpha = -1.5 .
The resulting mass-to-light ratio implies that the majority of sources responsible for reionization are too faint to be observed by the current surveys .