Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program that is capable of numerically evaluating QCD virtual corrections to scattering amplitudes. A detailed presentation of the algorithm is given, along with instructions to run the code and benchmark results. The program is part of the HELAC-NLO framework that allows for a complete evaluation of QCD NLO corrections. Program title:HELAC-1LOOP Catalogue identifier: AEOC_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEOC_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 290945 No. of bytes in distributed program, including test data, etc.: 3013326 Distribution format: tar.gz Programming language: Fortran (gfortran(http://gcc.gnu.org/fortran/), lahey95 (http://www.lahey.com), ifort3(http://software.intel.com)). Computer: Any. Operating system: Linux, Unix, Mac OS. Classification: 11.1. Nature of problem: The evaluation of virtual one-loop amplitudes for multi-particle scattering is a long-standing problem 1. In recent years the OPP reduction technique 2 opened the road for a fully numerical approach based on the evaluation of the one-loop amplitude for well-defined values of the loop momentum. Solution method: By using HELAC 3–5 and CutTools 6, HELAC-1LOOP is capable of evaluating QCD virtual corrections 7. The one-loop n-particle amplitudes are constructed as part of the n+2 tree-order ones, by using the basic recursive algorithm used in HELAC. A Les Houches Event (LHE) file is produced, combining the complete information from tree-order and virtual one-loop contributions. In conjunction with real corrections, obtained with the use of HELAC-DIPOLES 8, the full NLO corrections can be computed. The program has been successfully used in many applications. Running time: Depending on the number of particles and generated events from seconds to days. References: 1R.K. Ellis, Z. Kunszt, K. Melnikov and G. Zanderighi, arXiv:1105.4319hepph.2G. Ossola, C. G. Papadopoulos and R. Pittau, Nucl. Phys. B 763 (2007) 147 arXiv:hep-ph/0609007.3A. Kanaki and C. G. Papadopoulos, Comput. Phys. Commun. 132 (2000) 306 arXiv:hep-ph/0002082.4C. G. Papadopoulos, Comput. Phys. Commun. 137 (2001) 247 arXiv:hepph/ 0007335.5A. Cafarella, C. G. Papadopoulos and M. Worek, Comput. Phys. Commun. 180 (2009) 1941 arXiv:0710.2427 hep-ph.6G. Ossola, C. G. Papadopoulos and R. Pittau, JHEP 0803 (2008) 042 arXiv:0711.3596 hep-ph.7A. van Hameren, C. G. Papadopoulos and R. Pittau, JHEP 0909, 106 (2009) arXiv:0903.4665 hep-ph.8M. Czakon, C. G. Papadopoulos and M. Worek, JHEP 0908, 085 (2009) arXiv:0905.0883 hep-ph.