Event Sourcing is an architectural pattern that models system state as an immutable sequence of events, persisting the fact of every state change, with the current state derived by replaying all historical events in order; its advantages include a complete audit log, the ability to rebuild the state at any point in time, and the ability to fix projections or add new read models through replay, while the cost is that querying the current state requires aggregating events, and it is usually combined with CQRS and materialized views to optimize read performance.