  We describe AlterBBN, a public C program for evaluating the abundances of the
elements generated by Big-Bang nucleosynthesis (BBN). This program enables the
user to compute the abundances of the elements in the standard model of
cosmology, and additionally provides possibilities to alter the assumptions of
the cosmological model in order to study their consequences on the abundances
of the elements. In particular the baryon-to-photon ratio and the effective
number of neutrinos, as well as the expansion rate and the entropy content of
the Universe during BBN can be modified in AlterBBN. Such features allow the
user to test the cosmological models by confronting them to BBN constraints. A
presentation of the physics of BBN and the features of AlterBBN is provided
here under the form of a manual.
