Water contains constituents that significantly influence its properties. Although water supports life on earth there are certain constituents that may render it unsuitable for beneficial use—both for human consumption, as well as for industrial uses. Proper characterization is necessary to devise appropriate treatment systems for the use of water in various applications. Wastewater, on the other hand, is traditionally viewed as a liability to a utility or industrial facility due to the health and environmental threat it poses to the receiving bodies of water. Proper treatment and disposal methods can minimize most of the risks. However, wastewater can also perform the role of an asset when the treated effluent is reused in various applications, particularly in the water-stressed regions of the world. To better understand the characteristics of water and wastewater associated with various facets of petroleumPetroleum industry, it is essential to understand an extensive range of parameters, both biotic and abiotic, so that proper treatment schemes can be developed and implemented. A description of some of the important parameters, their effects, sources, and their detection methods form the basis of this chapter. It covers a wide range of subjects including water chemistry and microbiologyMicrobiology, chemical thermodynamics, and chemical and biochemical reaction kinetics, each of which is a subject by itself. For an in-depth understanding, interested readers are referred to the books listed at the end of this chapter as “Suggested Reading”.