PDA Technical Report 13 (Revised): Fundamentals of an Environmental Monitoring Program serves as a resource on controlled environmental test methods, and although some nonviable particulate information is included, the report's primary focus is microbiological control for sterile product manufacturing. PDA first published a guidance on environmental monitoring in the form of Technical Report No. 13 in 1990 and revised the guidance in 2001. This is the second revision of that guidance. This revision updates microbiological and particulate control concepts and principles as they relate to facilities involved in the manufacture of sterile pharmaceutical products and other designated controlled environments. It expands on PDA's 2001 revision of Technical Report No. 13 to reflect substantial changes to regulatory guidelines, international standards, and scientific advances in environmental monitoring procedures and equipment.