The Development of Science-based Guidelines for Laboratory Animal Care is the summary of an international workshop held in Washington, DC, in November 2003 to bring together experts from around the world to discuss the available knowledge that can positively influence current and pending guidelines for laboratory animal care, identify gaps in that knowledge in order to encourage future research endeavors, and discuss the scientific evidence that can be used to assess the benefits and costs of various regulatory approaches affecting facilities, research, and animal welfare.