Estoppel is a legal principle in both common and civil law jurisdictions. It prevents people from breaking their agreements, or prevents someone from denying what they have already said or done. By invoking estoppel, one party is stopped (or "estopped") from making certain legal arguments that contradict what was previously agreed upon or said.

Collateral estoppel, also known as issue preclusion, prevents relitigation of facts or legal arguments which were already established in a prior legal proceeding. This occurs when those facts or legal arguments were essential to the prior decision. Collateral estoppel applies even if the prior court determined the facts incorrectly, so long as both proceedings involved the same parties and the exact same issues.

Equitable estoppel is another form of estoppel and prevents a party from relying on a legal defense that is contrary to the facts that were established during earlier proceedings. This estoppel requires that the party misrepresented or concealed information in the past, and that the party relying on the estoppel detrimentally relied on the misrepresentation or concealment and did not have the ability to discover the truth.

Promissory estoppel occurs when a party materially changes his position in reliance on a promise made by another party. This promise is usually written, but may also be verbal or implied. For the doctrine to apply, the promisor's conduct must be either wrongful, or it must be within the scope of some other legal doctrine that prevents the party from going back on the promise. This type of estoppel prevents the promisor from revoking a promise they made in the past.

Estoppel certificates are commonly used when a landlord wants to conduct a transaction with a third party, such as a lender. To ensure that the tenants are aware of any changes to the agreement and that no claims exist that would prevent the transaction from going through, the tenant must sign an estoppel certificate.

The doctrine of estoppel is an important legal tool to ensure that parties adhere to their promises and abide by decisions established in legal proceedings. The various forms serve to protect parties in different legal contexts and protect against the unfair consequences of one party going back on their word.