[en] Environmental surveillance and monitoring are a critical part of an overall strategy for protection of the public and environment. For site closure and disposal activities, monitoring and surveillance can be expected to continue many years into the future. In the case of environmental monitoring, two general types are commonly discussed: compliance and performance (or functional) monitoring. The objective of compliance monitoring is relatively clear. Compliance monitoring is conducted to demonstrate compliance with specific regulatory limits (e.g., maximum contaminant levels in groundwater). In this case, if your measurement is above these values, it is a regulatory concern. Monitoring can be especially challenging for engineered solutions involving robust barriers expected to last for decades or potentially hundreds or thousands of years. Although some gradual releases are expected to occur as barriers lose effectiveness, observable changes in groundwater are likely to be very small and may be delayed for very long times. For robust engineered systems, compliance in the near future is not expected to be a significant concern. The question becomes, what type of monitoring can be used to identify the potential for a compliance problem in the future? This has led to increasing use of performance monitoring as an early warning system. Rather than comparing against environmental standards, performance monitoring is compared against assumptions and results from a performance assessment. Performance monitoring is used to observe other potential indicators of changes in system behavior (vadose zone concentrations, changes in chemistry near the waste, changes in engineered features). Translating a change in a local measurement to a potential change in the concentration in an aquifer that would lead to a potential compliance concern in the future is a significant challenge for performance monitoring. Such monitoring needs to be more carefully interpreted and communicated than compliance monitoring. Considerations for implementation of performance monitoring are the focus of this paper. (authors)