Achieving a precise description of multi-parton final states is crucial for
many analyses at LHC. In this contribution we review the main features of the
HELAC-NLO system for NLO QCD calculations. As a case study, NLO QCD corrections
for tt + 2 jet production at LHC are illustrated and discussed.Comment: 7 pages, 4 figures. Presented at 10th DESY Workshop on Elementary
Particle Theory: Loops and Legs in Quantum Field Theory, Worlitz, Germany,
April 25-30, 201