Gluonic effects in eta and eta-prime physics

We review the theory and phenomenology of the axial U(1) problem with emphasis on the role of gluonic degrees of freedom in eta and eta' production processes, especially the low-energy pN -> pN eta and pN -> pN eta' reactions.Comment: 20 pages, 2 figures, Invited talk at the WASA Workshop on Eta Physics, October 2001, Uppsala, Swede