We discuss the chemical evolution of metal poor galaxies and conclude that their oxygen deficiency is not due to: the production of black holes by massive stars or a varying slope of the Initial Mass Function, IMF, at the high-mass end. A varying IMF at the low-mass end alone or in combination with: (a) an outflow of oxygen-rich material, (b) an outflow of well-mixed material, and (c) the presence of dark matter that does not participate in the chemical evolution process, is needed to explain their oxygen deficiency. Outflow of material rich in oxygen helps to account for the large helium to oxygen mass ratio values derived from these objects, but it works against explaining the carbon to oxygen mass ratio and the heavy elements minus carbon and oxygen to oxygen mass ratio values.

26 pages, plain Tex, a postscript version with the figures included is available at ftp://132.248.1.7/papers/chem_evolution/irr_bcg.tar ; ApJ accepted