Water quality and water supply are essential for organic greenhouse grown crops to prevent soil contamination by undesirable chemicals and microorganisms, while providing the correct amount of water required for plant growth. The absence of natural precipitation combined with higher evapotranspiration due to higher temperature and longer cropping period requires an adequate supply of water. Water quality is commonly defined by its chemical, physical, and biological attributes. It is closely linked to the soil/rock native components, surrounding environment and land use. The runoff from urban, industrial, farming, mining, and forestry activities also significantly affects the quality of water available for greenhouse horticulture. High water quality is particularly important to prevent soil salinization and ensure optimal soil biological activity. Indeed, unbalanced organic fertilizer inputs may contribute to soil salinity, while soil microbial activities responsible for nutrient mineralization, soil suppressiveness and plant health, are affected by soil pH, ions, and contaminants. Poor water quality can also result in drip and micro irrigation clogging, plant toxicity, and product contamination by human pathogens or illicit compounds.