Event Sourcing with MongoDB is a way to think about domain objects and transaction processing by persisting an immutable log of deltas, or "domain events", in the database. This approach allows for the derivation of an object's state at any point in the past by replaying the event history sequentially. Event Sourcing challenges traditional notions of transaction processing while being a mature pattern with a long history. It is used across various use cases including database engines and financial systems, providing a flexible and scalable solution for managing complex data.