Chlorination continues to be one of the most effective and popular method of microfouling and macrofouling control worldwide. For this reason, we devote an entire chapter to this control method. We will examine the behavior of chlorine in water, the different treatment strategies available, the principles of designing a problemfree chlorine delivery system, and the environmental impacts of chlorine. We will also examine the effect of chlorine on materials of construction, the dosage and consumption calculations, and the economics of chlorination.