  We measure the star formation efficiency (SFE), the star formation rate per
unit gas, in 23 nearby galaxies and compare it to expectations from proposed
star formation laws and thresholds. We use HI maps from THINGS and derive H2
maps from HERACLES and BIMA SONG CO. We estimate the star formation rate by
combining GALEX FUV maps and SINGS 24 micron maps, infer stellar surface
density profiles from SINGS 3.6 micron data, and use kinematics from THINGS. We
measure the SFE as a function of: the free-fall and orbital timescales;
midplane gas pressure; stability of the gas disk to collapse (including the
effects of stars); the ability of perturbations to grow despite shear; and the
ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly
constant at 5.25 +/- 2.5 x 10^(-10) yr^(-1) (equivalent to an H2 depletion time
of 1.9x10^9 yr) as a function of all of these variables at our 800 pc
resolution. Where the ISM is mostly HI, on the other hand, the SFE decreases
with increasing radius in both spiral and dwarf galaxies, a decline reasonably
described by an exponential with scale length 0.2-0.25 r_25. We interpret this
decline as a strong dependence of GMC formation on environment. The ratio of H2
to HI appears to be a smooth function of radius, stellar surface density, and
pressure spanning from the H2-dominated to HI-dominated ISM. The radial decline
in SFE is too steep to be reproduced only by increases in the free-fall time or
orbital time. Thresholds for large-scale instability suggest that our disks are
stable or marginally stable and do not show a clear link to the declining SFE.
We suggest that ISM physics below the scales that we observe - phase balance in
the HI, H2 formation and destruction, and stellar feedback - governs the
formation of GMCs from HI.
