  The Collins fragmentation function describes a left/right asymmetry in the
fragmentation of a transversely polarized quark into a hadron in a jet. Four
different model calculations of the Collins function have been presented in the
literature. While based on the same concepts, they lead to different results
and in particular to different signs for the Collins function. The purpose of
the present work is to review the features of these models and correct some
errors made in previous calculations. A full study of the parameter dependence
and the possible modifications to these models is beyond the scope of the
paper. However, some general conclusions are drawn.
