BACKGROUND Researchers and policymakers increasingly identify active living-including walking and bicycling for travel and recreation-as a potential strategy to increase rates of physical activity in the United States. Understanding the impact of the built environment on physical activity levels requires reliable methods to measure potentially relevant built environment features. This paper presents an audit tool-the Irvine Minnesota Inventory-that was designed to measure a wide range of built environment features that are potentially linked to active living, especially walking. METHODS The inventory was created through a literature review, focus group interviews, a panel of experts, and field testing in 27 settings. The inventory was developed in 2003-2004. RESULTS The Irvine Minnesota Inventory includes 162 items, organized into four domains: accessibility (62 items), pleasurability (56 items), perceived safety from traffic (31 items), and perceived safety from crime (15 items). (Some items are in multiple domains.) The inventory includes both a paper version and a version in Microsoft Access, to allow data to be input directly into the computer. CONCLUSIONS Limitations of methods used to develop the inventory are discussed. Strategies are offered for using the Irvine Minnesota Inventory to systematically and reliably measure characteristics of the built environment that are potentially linked to active living.