Family law is simultaneously moving toward and away from formalist decision making. Examining family law across its various component doctrines—custody disputes, child support, jurisdiction, and parentage—reveals these two competing trends. In some of these areas, scholars and lawmakers have recognized that litigating under open-ended, amorphous standards is unpredictable and often painful, with costs that undermine the very purposes served by these legal frameworks; in these areas we are witnessing a turn toward determinate rules over judicial discretion as the preferred means of resolving disputes. In other areas, however, family law is experiencing a trend toward more flexible decision making that prioritizes functional assessment of relationships above formal legal status. This Article brings all of these developments into the same frame for the first time. It asserts that the “functional turn” in some areas of family law would benefit from the lessons learned in other areas: that indeterminate standards and contextualized decision making do not necessarily provide the best means of doing justice for separating families. I argue instead for a new formalism, one that extends the profound advantages of certainty and stability to a wider range of family relationships.