We interviewed measurement specialists at leading companies and interactive agencies to determine how they select the right Web site performance metrics. Through our analysis of their responses, we identified a three-step process for defining key performance indicators (KPIs): 1) Clarify site business goals; 2) identify critical tactics; and 3) choose indicative metrics. We recommend starting with the metrics that can be generated with the data that's currently available. Over time, companies should plan to automate additional reports and integrate the data sources required to produce the full complement of Web site KPIs.