Chapter One provides an overview of the history, purpose and objectives of the National Environmental Laboratory Accreditation Conference (NELAC). The organizational structure and function of NELAC, and the roles of the various participants, form the major portion of this chapter. In addition, the Constitution and Bylaws, and the content of the five chapters which follow are briefly described. Together, these six chapters and related appendices constitute the NELAC standards. Chapter 2 contains the criteria for the proficiency testing (PT) program. Chapter 3 describes the essential elements that are to be included in an on-site assessment and the requirements for an accrediting authority conducting on-site assessments. The qualifications and requirements for assessors are described as well as the program elements to ensure uniform and consistent implementation of the NELAC standards. Chapter 4 describes the accreditation process the laboratory must follow to be recognized as a NELAC laboratory. The chapter defines the period of accreditation, and the process for maintaining, awarding and revoking accreditation. Chapter 5 and the related appendices contain the elements of the laboratory quality system. Chapter 6 defines the process and operating requirements established by NELAC for an accrediting authority to become nationally recognized. The Glossary, which is contained as Appendix A to Chapter 1, contains the definition of terms which are used throughout the standards to assure the consistency of their use and interpretation.