Data, information and knowledge on the local groundwater systems and their context are indispensable for effective groundwater resources management, thus they form basic components of good groundwater governance. Therefore, this chapter briefly addresses the following subjects: (i) the role of data, information and knowledge, and how it varies according to actor and activity; (ii) the categories of data and information that are most relevant; (iii) mechanisms and provisions for generating data, information and knowledge; (iv) presenting, sharing and disseminating data and information; and (v) diagnostics as an important step from understanding groundwater systems towards decision-making on groundwater resources management. In spite of unprecedented progress made since the middle of the 20th century, the current availability of data, information and knowledge on the local groundwater conditions is in many parts of the world still insufficient for reliable diagnostics and optimal decisions on groundwater resources management policy and measures. Here lies a challenge for the entire groundwater community to mobilize support from politicians and other decision-makers for investing structurally and permanently in essential data, information and knowledge on their valuable groundwater systems.