The CAP Theorem, proposed by Eric Brewer, states that in a distributed data store, it is impossible to simultaneously guarantee all three of the following properties: Consistency, Availability, and Partition Tolerance. Consistency means that all nodes see the same data at the same time, while availability ensures that every request receives a response, even if some nodes are down. Partition tolerance allows the system to continue operating despite network failures. In practice, systems must choose to prioritize two of these three properties. For example, a system might be consistent and available but not tolerate network partitions, or it might be available and partition-tolerant but sacrifice consistency. Understanding the CAP Theorem helps developers make informed decisions about system design and trade-offs.