These guidelines for biosafety laboratory competency outline the essential skills, knowledge, and abilities required for working with biologic agents at the three highest biosafety levels (BSLs) (levels 2, 3, and 4). The competencies are tiered to a worker’s experience at three levels: entry level, midlevel (experienced), and senior level (supervisory or managerial positions). These guidelines were developed on behalf of CDC and the Association of Public Health Laboratories (APHL) by an expert panel comprising 27 experts representing state and federal public health laboratories, private sector clinical and research laboratories, and academic centers. They were then reviewed by approximately 300 practitioners representing the relevant fields. The guidelines are intended for laboratorians working with hazardous biologic agents, obtained from either samples or specimens that are maintained and manipulated in clinical, environmental, public health, academic, and research laboratories.