CERCLA Section 107(a) defines four classes of potentially responsible parties: the current owner and operator of a contaminated facility, any person who owned or operated the facility at the time hazardous substances were disposed of, any person who arranged for disposal or treatment of hazardous substances at the site, and any person who transported hazardous substances to the site and selected it for disposal. A party can fall into more than one class, and liability attaches even to parties who no longer own or have any relationship to the site today.

PRP liability is strict: the government or a private cost-recovery plaintiff does not need to prove negligence, only that the party fits one of the four statutory categories and that a release or threatened release of a hazardous substance caused response costs to be incurred. Liability is also retroactive, reaching conduct that was lawful when it occurred, and it can be joint and several when the harm is not reasonably divisible. The narrow statutory defenses — act of God, act of war, and act or omission of an unrelated third party — rarely succeed on their own.

PRP litigation routinely spans decades of ownership history and dozens of named and unnamed parties, which makes the eventual allocation of cleanup cost highly uncertain even after liability is established. In a Juricratic simulation, each PRP's exposure, contribution share, and settlement posture can be modeled as separate dials so a party can rehearse how allocation negotiations and contribution litigation might unfold under different assumptions about causation and equitable factors — a simulation artifact, not a prediction of any court's or agency's actual allocation.

PRP status is typically triggered when EPA or a state agency issues a general notice letter identifying a party as connected to a Superfund site, or when a private party who has incurred response costs sues to recover them. Named PRPs commonly respond by negotiating a consent order or settlement with the agency, then filing third-party or contribution claims against additional parties they believe share responsibility, producing sprawling multi-party litigation over factual causation and equitable allocation.