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This chapter describes assessment, which is a one-time process that relies on a statistically valid sampling within a region and monitoring that consists of a series of measurements designed to provide information on the trajectory of change in the ecosystem structure and/or function over time. An assessment system provides data on the status of the ecosystems of a region and should be used across broad geographical areas. The measurements made in an assessment system must be rapid and sufficiently simple to be made by the persons with limited experience and expertise. Assessment measurements are converted into indicators by combining a group of measurements or by calculating indicator values from the basic measurements. Indicators should be measures of the ecosystem properties that are related to ecosystem processes in some documented way. If assessments are made repeatedly at the same locations, the assessment indicators can be incorporated into a monitoring system to ascertain the trends in the ecosystems of a region. Monitoring sites should be selected on the basis of a set of criteria or questions that may differ from region to region and even among locations within a region. The site locations and the measurements selected should be chosen to answer one or more questions about the status and trends of the ecosystems of a landscape unit.