ONELOOP is a program to evaluate the one-loop scalar 1-point, 2-point, 3-point and 4-point functions, for all kinematical configurations relevant for collider-physics, and for any non-positive imaginary parts of the internal squared masses. It deals with all UV and IR divergences within dimensional regularization. Furthermore, it provides routines to evaluate these functions using straightforward numerical integration. Program summaryProgram title: OneLOop Catalogue identifier: AEJO_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEJO_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.: 12 061 No. of bytes in distributed program, including test data, etc.: 74 163 Distribution format: tar.gz Programming language: Fortran Computer: Workstations Operating system: Linux, Unix RAM: Negligible Classification: 4.4, 11.1 Nature of problem: In order to reach next-to-leading order precision in the calculation of cross sections of hard scattering processes, one-loop amplitudes have to be evaluated. This is done by expressing them as linear combination of one-loop scalar functions. In a concrete calculation, these functions eventually have to be evaluated. If the scattering process involves unstable particles, consistency requires the evaluation of these functions with complex internal masses. Solution method: Expressions for the one-loop scalar functions in terms of single-variable analytic functions existing in literature have been implemented. Restrictions: The applicability is restricted to the kinematics occurring in collider-physics. Running time: The evaluation of the most general 4-point function with 4 complex masses takes about 180 μs, and the evaluation of the 4-point function with 4 real masses takes about 18 μs on a 2.80 GHz Intel Xeon processor.