  We present the library Collier for the numerical evaluation of one-loop
scalar and tensor integrals in perturbative relativistic quantum field
theories. The code provides numerical results for arbitrary tensor and scalar
integrals for scattering processes in general quantum field theories. For
tensor integrals either the coefficients in a covariant decomposition or the
tensor components themselves are provided. Collier supports complex masses,
which are needed in calculations involving unstable particles. Ultraviolet and
infrared singularities are treated in dimensional regularization. For soft and
collinear singularities mass regularization is available as an alternative.
