In this chapter, we discuss several types of quality assurance (QA) and quality control (QC) measures, which are described for the three main stages in the field survey in sequence. This comprises the process of aerial-image interpretation, which is crucial for ensuring the complete coverage of the Swiss forest in the field sample. We also discuss the QA activities applied during fieldwork, including the training of field teams, the maintenance of the field manual, and several organisational measures to prevent mistakes. Finally we describe the additional surveys conducted to assess the quality (reproducibility) of the field assessments in terms of precision (uncertainty) and accuracy, and summarise the quantitative results of selected attributes from the Swiss National Forest Inventory (NFI)'s repeat survey.