  It is widely believed that the cosmic baryon density may be obtained by
inferring deuterium abundances in low-metallicity quasar absorption line
systems. The implicit assumptions which enter this argument are critically
assessed. In particular, the production of deuterium in non-standard Big Bang
nucleosynthesis scenarios, the putative production of deuterium in
astrophysical environments, and the possible destruction of deuterium via
non-standard chemical evolution are discussed.
