Chapter 6 discusses building encroachment, which is a topic closely related to adverse possession—at least in the common law. Chapter 6 documents the three different doctrinal approaches to dealing with encroachment over boundary and focuses on the building encroachment doctrine enacted in 52 jurisdictions. The prompt protest rule and encroachers’ not acting in bad faith are easy to justify economically (though not universally adopted). Encroachers’ good faith is increasingly unlikely given the advance of mapping technology. A two-tier building encroachment doctrine (with safe harbor and sure shipwreck) is best. Even though this doctrine has been used as an example of a put option, it is not, and will be inefficient if treated as such.