This document proposes an approach for intrusion detection that uses clustering and classification. In the first phase, the K-means clustering algorithm is used to generate clusters of similar attack types from network/system activity data streams. The centroids generated by K-means are then used in the second classification phase, where the K-nearest neighbors and decision tree algorithms detect the different types of attacks present in the data. The overall goal is to take advantage of clustering to group similar attacks first before classifying them, which can provide better detection performance than using a single classifier alone.