A number of questions relative to the background of hydrology are discussed. What does the term “water” mean in a scientific context? Why is there water on our planet? Why is most of it in liquid form? What does science tell us about the molecular structure of liquid water? Can liquid water be treated as a continuum? Is water a Newtonian fluid? What is the scientific basis for the equations of water movement commonly used in hydrology? Can the various approaches to the study of water movement be reconciled with one another? What does the concept of water mean to the physical scientist, the biological scientist, the social scientist and above all to the hydrologist?