A thorough understanding of one-dimensional flows provides a solid foundation for more complex multidimensional problems. If the flow is also isentropic, the governing equations can be integrated and the flow patterns can be studied using Riemann invariants. In the first half of this chapter, we will focus on flows resulting from the motion of a piston in a long tube of constant cross section. In the second half, we will consider flows in ducts with small variations in cross section.