Predictive emissions monitoring systems (PEMS) are recognized as potentially acceptable monitoring methods and an alternative to hardware continuous emissions monitoring systems (CEMS) for demonstrating continuous compliance. Upcoming state and federal regulations are driving forces for regulatory agencies to develop standardized PEMS protocols for specific source categories and processes. PEMS is a promising technology whose use in the field remains to be demonstrated. For the first time, practical issues relating to accuracy, reliability, and quality assurance of these systems are surfacing. This paper provides an update of the current understanding and regulatory status of PEMS, and addresses practical issues and concerns relating to its quality assurance and field implementation.