  We explore the effect of neutron lifetime and its uncertainty on standard
big-bang nucleosynthesis (BBN). BBN describes the cosmic production of the
light nuclides $^1{\rm H}$, ${\rm D}$, $^3{\rm H}$+$^3{\rm He}$, $^4{\rm He}$,
and $^7{\rm Li}$+$^7{\rm Be}$ in the first minutes of cosmic time. The neutron
mean life $\tau_n$ has two roles in modern BBN calculations: (1) it normalizes
the matrix element for weak $n \leftrightarrow p$ interconversions, and (2) it
sets the rate of free neutron decay after the weak interactions freeze out. We
review the history of the interplay between $\tau_n$ measurements and BBN, and
present a study of the sensitivity of the light element abundances to the
modern neutron lifetime measurements. We find that $\tau_n$ uncertainties
dominate the predicted $^4{\rm He}$ error budget, but these theory errors
remain smaller than the uncertainties in $^4{\rm He}$ observations, even with
the dispersion in recent neutron lifetime measurements. For the other
light-element predictions, $\tau_n$ contributes negligibly to their error
budget. Turning the problem around, we combine present BBN and cosmic microwave
background (CMB) determinations of the cosmic baryon density to
$\textit{predict}$ a "cosmologically preferred" mean life of $\tau_{n}({\rm
BBN+CMB}) = 870 \pm 16 \ \rm sec$, which is consistent with experimental mean
life determinations. We go on to show that if future astronomical and
cosmological helium observations can reach an uncertainty of $\sigma_{\rm
obs}(Y_p) = 0.001$ in the $^4{\rm He}$ mass fraction $Y_p$, this could begin to
discriminate between the mean life determinations.
