This article studies the promulgation of law in Qin and Western Han China (221 b.c.e.–9 c.e.) based primarily on excavated legal and administrative texts. It shows that a new law was handed down from the emperor to the relevant offices on the day of enactment. The article argues that, to an extent, the subject matter and function of a law determined for whom it was passed and promulgated. Depending upon the location, rank, and official duties of the offices, the laws known and used could be quite different. Although it was required that documents of imperial decisions be forwarded swiftly and safely by courier at the prescribed speed, delays in forwarding such documents to distant local offices were probably common in Qin and Western Han China. Evidence indicates that district- and prefecture-level officials publicized laws that needed to be made known by the common people, by reading them aloud in local gatherings, for example, or posting them in conspicuous places. The article further argues that a law came into effect in offices on the day it arrived at local courts or on the day it was enacted in the central court, depending on the existence of related extant laws. It concludes that a new law in Qin and Western Han China was ex post facto, as it reached backwards to a past action and retroactively attached liabilities to the action at the point when it was performed.