  We consider the formation of OB associations from two perspectives: (a) the
fractional gas consumption in star formation,epsilon, per dynamical time scale
t_dyn in a galaxy, and (b) the origin of the so-called Kennicutt-Schmidt law
that the rate of star formation per unit area is proportional to a power,
alpha, of the surface density in HI and H_2 gas when certain thresholds are
crossed. The empirical findings that epsilon is approximately 0.01 and alpha is
approximately 1.4 or 1.5 have simple explanations if the rate of star formation
is magnetically regulated. An empirical test of the ideas resides in an
analysis of why giant OB associations are ``strung out like pearls along the
arms" of spiral galaxies.
