The purpose of this book is to assist institutions in caring for and using animals in ways judged to be scientifically, technically and humanely appropriate. The book is also intended to assist investigators in fulfilling their obligation to plan and conduct animal experiments in accordance with the highest scientific, humane and ethical principles. Recommendations in the book are based on published data, scientific principles, expert opinions and experience, with methods and practices that have proved to be consistent with both high-quality research and humane animal care and use. These recommendations should be used as a foundation for the development of a comprehensive 'animal care and use' program, recognizing that the concept and application of performance standards, in accordance with goals, outcomes and considerations defined in the book, are essential to this process. In approaching its task, the team carried forward the balance between ethical and science-based practice that has always been the basis of the book, and fulfilled its role to provide an updated resource that enables the research community to proceed responsibly and in a self-regulatory manner with animal experimentation. The book is predicated on the understanding that the exercise of professional judgment both upholds the central notion of performance standards and obviates the need for more stringent regulations. Laboratory animal science is a rapidly evolving field and the team identified a number of areas in which current available scientific information is insufficient; additional objective information and assessment are needed to provide a scientific basis for recommend-ations in future editions of the book. Although pursuing these concepts was beyond this team's charge, the following two topics merit further study: (1) space and housing needs of laboratory species, and (2) the need and best methods for providing enrichment, exercise and human contact.