Joseph Silk (Johns Hopkins Univ. Baltimore/Univ. Pierre et Marie Curie, Paris) gives a talk at the Workshop on "Why trust a Theory?" (7-9 December, 2015) titled "The Limits of Cosmology, Post-Planck". Abstract: I will discuss how one might follow up on the Planck satellite which has given us a remarkable confirmation to high precision of what has become known as the “standard model of cosmology.” This model is purely phenomenological and establishes a robust framework around which a number of fundamental issues remain unresolved. In order to make further progress, what is our optimal choice of future strategy?