This document discusses efficient data stream classification using probabilistic adaptive windows. It introduces the concept of data streams which have potentially infinite sequences of high-speed data that must be processed in real-time with limited memory. It then describes the probabilistic approximate window (PAW) algorithm, which maintains a sample of data instances in logarithmic memory by giving greater weight to newer instances. The document evaluates several data stream classification methods on real and synthetic data streams and finds that k-nearest neighbors with PAW has higher accuracy and lower memory usage than other methods.