Owing to the fact that a Living Reviews in Relativity article may evolve over time, we recommend to cite the article as follows:
Luca Amendola and Euclid Theory Working Group,
"Cosmology and Fundamental Physics with the Euclid Satellite",
Living Rev. Relativity 16, (2013), 6. URL (accessed <date>):
http://www.livingreviews.org/lrr-2013-6
The date given as <date> then uniquely identifies the version of the article you are referring to.