Groundwater is sometimes called “the hidden asset” – awareness of its existence and its importance is not well known and as a consequence the measures which are required to protect and manage it in an environmental sustainable way are either not taken or are taken too late. Where pollution has occurred and measures are taken too late it may take decades, or longer, until the necessary restoration of quality is achieved.This comprehensive text presents in the following sections:Groundwater monitoring in the regulatory and international contextConceptual modelling and network designGroundwater pollutants and other pressuresGroundwater quality standards and trend assessmentCase studies for groundwater assessment and monitoring in the light of EU legislationGroundwater measurementsAssociating external stakeholdersThe editors have collected state-of-the-art information on groundwater quality assessment monitoring from the international community, providing further stimulation to the work of all parties involved in the challenges this area creates to ensure sound quality assessment of groundwater.