Abstract: Property has a complexity problem. Although both “property” and “complexity” are often invoked in property theory, we need more and better notions of both. Much theorizing about property law and institutions suffers from an excessive and misguided reductionism, what I call “Flatland.” The Flatland style of theorizing reduces law to a heap of rules and property to a merely additive bundle of rights. By incorporating complexity based on dense interaction into the picture, we can overcome some false dichotomies in property theory. These include the unstructured collection of “sticks,” the flattening of system, all-or-nothing formalism, misunderstandings of the role of information costs, and the assumption that purposes must be directly reflected in individual rules of property. By contrast, seeing system in property as a method of managing complexity points to the importance of exclusion versus governance; hybrid regimes of private, common, and public property; a spectrum of formalism including law versus equity; degrees of modularity and thing-ness; a combination of spontaneous and directed evolution; and a synergy of common law and legislation. Implementing these aspects of system and overcoming problematic reductionism and false dichotomies will require an encounter with practice. Applications to the law of possession, aerial trespass, nuisance, and the integration of property “bundles” demonstrate how theory can meet practice.