Clustering is a vital methodology in the data storage world. Its goal is to maximize cost effectiveness, availability, flexibility, and scalability. Clustering has changed considerably for the better due to Storage Area Networks, which provide access to data from any node in the cluster. It explains how clusters with shared storage work and the components in the cluster that need to work together. It reviews where a cluster should be deployed and how to use one for best performance. The author is Lead Technical Engineer for VERITAS Cluster File Systems and has worked on clusters and file systems for the past ten years.