Steam systems and steam condensate systems provide tragic examples of vapor collapse during fluid transients. Chapter 5 provided examples of vapor collapse during liquid/vapor fluid transients, where small quantities of vapor were present. When larger quantities of vapor are present in steam systems, the fluid mechanics of two-phase flow are far more complex. This chapter provides historical examples of water hammer, a discussion of two-phase transients (vapor collapse), a brief discussion of single-phase transients (blowdown), and some actions to prevent damages during steam system operation.