The design and construction of groundwater monitoring points is as important for groundwater monitoring purposes as it is for precautionary, sustainable groundwater protection. Only correctly designed, competently constructed and reliably operating monitoring points can ensure meeting the current requirements of groundwater quality monitoring. A long service life, safe functioning and, in particular, safe sealing, count among some of the important quality features of groundwater monitoring points. This Code of Practice W 121 describes how to competently design and construct groundwater monitoring points for monitoring the quantity and quality of groundwater. It describes the quality requirements for the construction, the building supervision and the acceptance of groundwater monitoring points.