The theory for predicting flow pattern transition under transient flow conditions is developed and compared with experiment. This work represents an extension of the methods presented by Taitel and Dukler (1976) for steady state flows. Under transient conditions, flow pattern transitions can take place at flow rates substantially different than would occur under steady flow conditions. In addition, flow patterns can appear which would not be expected for a slow change in flow rates along that same path. Methods are presented for predicting the flow rates at which flow pattern transitions will take place during flow transients. The method also reveals when spurious flow patterns will appear.