Water is intricately connected with all sectors of the petroleumPetroleum industry. In the upstream sector it plays a critical role in the production from unconventional sources by hydrofracturingHydrofracturing operation, and also in enhancing the secondary productionSecondary production from conventional sources. Another important consideration in the upstream business sector is the handling of large amounts of water produced from the explorationExploration of crude. This chapter discusses the water use and water treatmentWater treatment in the upstream and midstream segments of the petroleum industry. Included are discussions of the sources and quality of water used and produced in these segments, and the methods of treatment practiced in this industry for appropriate disposal and/or reuse. To understand the subject of any industrial water and wastewater treatment process it is essential to develop an understanding of the types of chemicals that are present in those streams. With that in view a brief description of the chemical composition and characteristics of petroleumPetroleum is presented in Appendix B.