The arbitrary lines of zoning maps across the country have come to dictate where Americans may live and work, forcing cities into a pattern of growth that is segregated and sprawling. The good news is that reform is in the air, with states across the country critically reevaluating zoning. In cities as diverse as Minneapolis, Fayetteville, and Hartford, the key pillars of zoning are under fire, with apartment bans being scrapped, minimum lot sizes dropping, and off-street parking requirements disappearing altogether. Some American cities already make land-use planning work without zoning. In Arbitrary Lines, Gray lays the groundwork for this ambitious cause by clearing up common confusions and myths about how American cities regulate growth and examining the major contemporary critiques of zoning. Gray sets out some of the efforts currently underway to reform zoning and charts how land-use regulation might work in the post-zoning American city. Gray shows how zoning has failed to address even our most basic concerns about urban growth over the past century, and how we can think about a new way of planning a more affordable, prosperous, equitable, and sustainable American city.