Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Timothy M. Adamo and Carlos Kozameh and Ezra T. Newman,
"Null Geodesic Congruences, Asymptotically-Flat Spacetimes and Their Physical Interpretation",
Living Rev. Relativity 12, (2009), 6. URL (accessed <date>):
The date given as <date> then uniquely identifies the version of the article you are referring to.