Providing food, water and a place to defecate safely have been the building blocks of humanitarian services in emergencies. Recently, incredible evolutions have occurred in the fields of water, sanitation, nutrition and environmental health. In order to understand how these areas interact, we must have a firm grasp on the basic logic behind water and sanitation interventions. It is likely that in the next crisis in which you work, diarrhea will be one of the top two killers of children. By engaging with this course material and obtaining a deep understanding of the basic provisions of water and sanitation, you and your work can help prevent those deaths and help people in crises remain healthy.