Given the relevance of brine chemistry, degassing, and mineral precipitation on the design, operating philosophy, maintenance requirement, efficiency and general performance of the geothermal plant, a good knowledge of the geothermal fluid's chemical composition and phase envelope is required. Thus, fluid sampling should be performed early in the project. The chemical analysis of the samples (e.g., brine, brine-gas mixtures) can then be fed into geochemical/aqueous speciation models, which not only allow predicting the extent of gas solubility in brines and the potential for scaling as pressure and temperature shift, but also provide essential input to corrosion models. Sampling should also be carried out periodically throughout the lifetime of the geothermal plant for monitoring purposes; any shift in the chemistry conditions identified during the monitoring routine can then be used to update the models, ensuring reliable predictions.