This standard W 129 describes methods and procedures for a professional inspection of the correct operation of groundwater monitoring points. The determination of reliable and representative groundwater monitoring data is of particular importance in order to meet the manifold demands on groundwater monitoring and in terms of preventive and sustainable groundwater protection. The regular performance of suitability tests at groundwater monitoring points makes a considerable contribution to quality assurance. Only functional groundwater monitoring points that are suitable for the respective task can meet the requirements of goal- and result-oriented groundwater monitoring. The standard provides practical recommendations for carrying out technical suitability tests for groundwater monitoring points. Flowcharts illustrate the sequence of the test steps to be performed. Important test procedures are explained in the annex.