Preface ASHRAE Laboratory Design Guide: Planning and Operation of Laboratory HVAC Systems is a significant revision of the first edition of the design guide@ which was published in 2001. It is intended that the information in this Guide assist engineers@ owners@ and system operators in determining the needs of their laboratory facility and how to best match these needs to mechanical system options. This Guide is written in a manner that progresses from general to specific to reach a wide target audience of designers@ architects@ engineers@ owners@ operation and maintenance personnel@ and others in the heating@ ventilating@ air-conditioning@ and refrigerating (HVACR) industry. The first chapters of the Guide present general background information and prescribed design frameworks@ whereas the later chapters are more specific@ providing detailed design and application information. A primary benefit of this structure is that many types of readers can use the Guide. Whereas an owner may only read the first two chapters@ an engineer may focus on the design chapters. This new edition has been updated to reflect current standards and industry practices. Web links to these standards and to additional laboratory-related resources have been expanded. Many of the chapters have been significantly revised or rewritten. This edition also contains two new chapters@ one that covers sustainability and one that covers computer modeling of ventilation effectiveness. It is hoped that this revised edition of the Guide will be even more useful to the laboratory designer@ owner and operator@ and anyone else involved in the design@ construction@ and operation of laboratories.