The effect of the liquid Froude number on the physical processes for forming slugs was investigated for cocurrent air-water flow through a 7.63 cm horizontal pipeline that was 20 m long. The transient characteristics are described by using simultaneous measurements of the liquid holdup at multiple locations. When the Froude number of the stratified flow at the onset of slugging is less than unity, the frequency is related to the propagation, upstream of gravity waves caused by the formation of slugs. A stochastic model is used to describe the formation for Froude numbers greater than unity. These results provide a physical basis for developing models for the frequency of slugging.