  The analysis of hyperon semi-leptonic decay data is addressed with reference
to SU(3) breaking and isospin mixing between Lambda^O and Sigma^O. Various
approaches to SU(3) breaking are discussed and compared. The phenomenological
implications of Lambda^0-Sigma^0 mixing are not to be underestimated: it can
induce vector couplings in decays otherwise purely axial and may also modify
rates. In regard of the KTeV data on Xi^0O -> Sigma^+ e antineutrino,
predictions are presented and the impact of present a future data on the
extraction of F and D is also examined. In addition, the implications of the
new data for the use of octet baryon beta decays in determining V_us are
considered.
