Parton Distribution Functions (PDFs) are a crucial input at the LHC, their uncertainty often being the limiting factor in the accuracy of theoretical predictions. At the same time the LHC is providing us with a number of precise measurements that help in constraining PDFs. 
In the first part of this talk a theoretical and experimental overview on the state of the art of PDF determination is given, with a particular emphasis on the theoretical aspects involved in determining the structure of the proton. In the second part, recent results from the NNPDF collaboration are presented and phenomenological implications for the high energy run of the LHC are explored.