The USP 797 requires sterility of all Compounded Sterile Products (CSP). Environmental Monitoring is an essential component to prove that a cleanroom, or clean device, meets the defined ISO 14644-1 classifications. These classifications ensure that a designated zone is maintained in a clean state. The monitoring program is designed based upon contamination risk to finished product quality. According to USP 797, critical areas must continuously meet ISO Class 5 or better conditions for 0.5 μm particles and must exclude microbial contamination during compounding of CSPs. An effective environmental monitoring program provides meaningful information on the quality of the compounding environment and any environmental trends in surrounding areas. In addition, an effective environmental monitoring program will identify potential routes of contamination, allowing for implementation of corrections to prevent CSP contamination.