A star formation law should predict the SFE from local conditions. Here we describe three proposals for the limiting timescale over which gas forms stars: the free-fall timescale in the gas disk, the orbital timescale, and the characteristic timescale for cloud-cloud collisions. We also describe proposals that GMCs form stars with a fixed SFE and that the midplane gas pressure regulates the fraction of the ISM in the molecular phase. We present each proposal as a prediction for the SFE in terms of observables. These appear together in the upper part of Table 1 .