Promissory Estoppel is a legal principle that prevents a party from withdrawing a promise when the other party has relied on that promise to their detriment. It typically arises in situations where a promise is made without a formal contract, yet the promisee takes action based on that promise, leading to potential unfairness if the promisor is allowed to back out. To establish Promissory Estoppel, three key elements must be present: a clear and definite promise, reliance on that promise by the other party, and resulting harm or detriment from that reliance. This doctrine aims to promote fairness and justice in situations where formal agreements may not exist.