  Using the approach proposed a few years ago by X. Ji, it has become feasible
to extract parton distribution functions (PDFs) from lattice QCD, a task
thought to be extremely difficult before Ji's proposal. In this talk, we
discuss this approach, in particular different systematic effects that need to
be controlled to ultimately have precise determinations of PDFs. Special
attention is paid to the analysis of excited states. We emphasize that it is
crucial to control excited states contamination and we show an analysis thereof
for our lattice data, used to calculate quasi-PDFs and finally light-cone PDFs
in the second part of this proceeding (C. Alexandrou et al., Quasi-PDFs from
Twisted mass fermions at the physical point).
