Nearly 20% of New York City land is dedicated to public parkland. As parcels of ground that are plotted, mapped, and potentially saleable in a market for real property, the public parks of New York City have remained overwhelmingly stable—on a parcel-by-parcel basis—for over a century. We do not observe the same durability in other forms of public land (e.g. school grounds), or in other forms of “sacred” land (e.g. cemeteries). The remarkable persistence of New York City parkland is the empirical puzzle at the center of this dissertation.,In New York State, municipal parkland is held in trust by the state for the public. Parks, as property, constitute a relationship between a diffuse public in perpetuity and the trustees of the public interest in parkland. To functionally protect parkland, this interest must be continually defined and defended. By anchoring the public/trustee relationship through indefinite time—over which “encroachments” accrue as precedent, while material parkland degrades as legible landscape architecture—preservationists construct the concept of parkland as constantly threatened with reversion to commodity land. Successful preservation techniques have disarticulated parks from land, offering justifications for the preservation of parks as e.g. art or scenic landmarks. Preservationists also construct “the threatened park” by framing proposals for the altered use of parkland as catering to restricted interests at the expense of a broadly construed public. ,Park preservation differs from historic building preservation, which in New York City has saved select gems while sacrificing the remaining built environment. Park preservationists instead defend a property relationship, in this way linking specific battles over parcels of land to constrained rules of exchange for the administrative category of parkland. Site-specific fights, particularly over the proper uses of Central Park, contribute to the category-level ideology of park purposes. This ideology, in turn, serves to maintain most parkland as practically precluded from exchange. The definition of what constitutes a “park purpose”—what can be permissibly built or temporarily allowed to occupy space within a city park—is the determining question in the adjudication of parkland alienation disputes in New York State courts, making this a fateful question for the shape of the city.