The law is permanently under construction. Most legal change is intentional. A legislator, a court, or one of the law's subjects hopes to better achieve a purpose by switching from one rule, one interpretation, or one remedy to the next. Yet empirically, legal innovation tends to be a process that takes time. At the macro level, the diffusion path is often S shaped: It does not start immediately and levels off after a while. This article links legal innovation to diffusion research and discusses micro processes that have the potential to generate the observed diffusion paths. Expected final online publication date for the Annual Review of Law and Social Science, Volume 18 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.