Metrology and its applications e.g. in chemical or food analysis or in environmental monitoring are entering our daily life. This book provides a basic overview over the relevant metrological concepts like traceability, ISO uncertainties or cause-and-effect diagrams. The applications described in great detail range from progression-of-error type evaluation of the measurement uncertainty budget to complex applications like pH measurement or speciation calculations for aqueous solutions. The consequences of a measurement uncertainty concept for chemical data are outlined for geochemical modeling applied to transport in the subsurface and to nuclear waste disposal. Special sections deal with the deficits of existing thermodynamic data for these applications and with the current position of chemical metrology in respect to other quality assurance measures, e.g. ISO 900x, GLP, European and U.S.-American standards.