When volcanoes are active, there are characteristic signs such as ground movement, sounds, heat and ejected material. Each of these signs is a result of, and hence an information source for, fluid motion in volcanic conduits. Here we briefly review some of the links between these signs and fluid flow processes and suggest future directions that should allow advancement of eruption forecasting as these links become understood more fully. Cross-fertilization between increasingly realistic numerical and experimental models, diverse geophysical data sources, and chemical and physical evidence in eruptive products can be achieved by simultaneously applying these approaches to well-studied volcanoes.