The Index Service of Globus Toolkit 3.0 is a helpful component for building grid applications. It can be used to index Service Data carrying state information from multiple grid service instances for use in resource discovery, selection and optimization. In this article the author explains how to set up the Index Service for static and dynamic indexing, and how to improve the reliability of the indexing. The primary way of writing a grid service with custom Service Data and how these data can be queried from the Index Service after aggregation are also covered.