A model for estimating void fraction in countercurrent bubbly and slug flows in vertical systems, based on the drift-flux approach, is presented. In countercurrent flow, as in cocurrent flow, the in situ gas velocity is aided in the upward direction by the buoyancy of the gas phase as well as its tendency to flow through the channel center. These two factors allow us to express the in situ gas velocity and the void fraction in terms of the bubble rise velocity, the mixture velocity, and the flow parameter, C0. Single bubble rise velocity data gathered for various falling liquid velocities agreed well with the Harmathy correlation. The parameter C0 in countercurrent bubbly flow was found to be 2.0, a value much higher than the generally accepted one of 1.2 for cocurrent flow. Significant liquid recirculation at the pipe wall during countercurrent bubbly flow, causing changes in the velocity and bubble concentration profiles similar to those observed in bubbly flow through stagnant liquid columns in large diameter pipes, is postulated to be the cause for this high value of C0. The Taylor bubble rise velocity data gathered at various falling liquid velocities agreed well with the Nicklin equation. Visual observation indicated little distortion in the shape of Taylor bubbles due to liquid flow in the opposite direction. Void fraction during slug flow is computed using two different approaches. Good agreement is found between theory and experiment.