This chapter examines the modular systems for energy usage in desalination and wastewater treatment. The availability of potable water is a global issue, and it is becoming more and more important. Furthermore, the process of desalination of sea or brackish water is very energy consuming. As shown by MIT study, the use of modular desalination process makes significant sense in many cases. The chapter first describes the available modular desalination processes. Subsequently the chapter describes the use of renewable energies such as solar, geothermal, biomass, and wind for modular desalination processes. The chapter also delineates the advantages and disadvantages of these renewable desalination technologies. Desalination using nuclear heat is also gaining worldwide acceptance, and the chapter reviews available processes in this regard. Small modular nuclear reactors are ideally suited for this purpose. Finally the chapter describes various commercially available modular systems for wastewater treatment which consumes a large portion of the public and private sectors’ operating budget.