QoS Levels define how reliably messages are delivered in IoT communication protocols like MQTT. QoS 0 means messages are delivered at most once, without confirmation. QoS 1 guarantees delivery at least once, but duplicates are possible. QoS 2 ensures exactly-once delivery, providing the highest reliability but also requiring more network resources. Choosing the right QoS level is a trade-off between reliability, bandwidth, and latency. In IoT, different use cases demand different QoS—for example, sensor updates may use QoS 0, while critical alerts may require QoS 2.