Monitoring systems rely heavily on online event detection. Understanding the distinction between online and offline scenarios is needed for event detection. Section 6.1 clarifies the differences between these two scenarios. Online detection has new problems not present in offline scenarios, including memory management, concept drift, model retraining, and model selection (Sect. 6.2). Section 6.3 discusses online prediction, a specialized form of online event detection. Section 6.4 presents advanced topics in online event detection.