Estoppel is a legal principle that prevents a party from asserting a claim or fact that contradicts a previous statement or agreement. Its primary goal is to ensure fairness and consistency in legal proceedings by preventing parties from deceiving or harming others who have relied on their earlier representations.

Estoppel originated in the early equitable jurisdiction of common law courts. Judges of equity developed it as a remedy to prevent injustices that would occur if strict adherence to common law rules was enforced in certain situations.

The three main forms of estoppel discussed are equitable estoppel, promissory estoppel, and judicial estoppel. Equitable estoppel arises from misleading conduct causing detrimental reliance. Promissory estoppel prevents reneging on a promise that induced detrimental reliance, even without a contract. Judicial estoppel prevents taking inconsistent positions in different legal proceedings.

The essential elements commonly found in estoppel claims are a representation (or conduct), reliance by the other party, and resulting detriment to the relying party. Reliance is significant because it demonstrates that the party acted based on the representation, and the potential for injustice arises if the original party can then contradict that representation.

Equitable estoppel arises when one party's misleading conduct or representations induce another party to act to their detriment. For example, if a landlord verbally assures a tenant that they can sublet their apartment, and the tenant then incurs costs finding a sublessee, the landlord might be estopped from later denying the tenant's right to sublet.

Promissory estoppel prevents a party from breaking a promise, even if there's no formal contract, if the promisee reasonably relied on that promise and suffered a loss as a result. It differs from a formal contract because it doesn't require consideration. It might be invoked when enforcing the promise is necessary to avoid injustice due to the promisee's detrimental reliance.

The purpose of judicial estoppel is to protect the integrity of judicial proceedings by preventing parties from "playing fast and loose" with the courts. Key requirements include the party having taken a specific position in a prior legal proceeding, that position being accepted by the court, and the party subsequently trying to assert a contradictory position in a later proceeding.

The main function of collateral estoppel, or issue preclusion, is to prevent the re-litigation of specific factual or legal issues that have already been conclusively decided in a prior legal proceeding involving the same parties.

One policy consideration underlying estoppel is fairness and justice. The doctrine aims to prevent opportunistic behavior by holding parties accountable for their representations when others have reasonably relied on them, thus promoting reliability and predictability in legal interactions.

One criticism of estoppel is that its rigid enforcement might sometimes lead to unjust results, particularly if the party making the initial representation did so under duress, without fully understanding the implications, or if subsequent circumstances have significantly changed.