  We discuss the lifetimes and evolution of dense cores formed as turbulent
density fluctuations in magnetized, isothermal molecular clouds. We consider
numerical simulations in which we measure the cores' magnetic criticality and
Jeans stability in relation to the magnetic criticality of their ``parent
clouds'' (the numerical boxes). In subcritical boxes, dense cores do not form,
and collapse does not occur. In supercritical boxes, some cores collapse, being
part of larger clumps that are supercritical from the start, and whose central,
densest regions (the cores) are initially subcritical, but rapidly become
supercritical, presumably by accretion along field lines. Numerical artifacts
are ruled out. The time scales for cores to go from subcritical to
supercritical and then collapse are a few times the core free-fall time,
$\tfc$. Our results suggest that cores are out-of-equilibrium, transient
structures, rather than quasi-magnetostatic configurations.
