A population balance equation is used to describe the evolution of bubble sizes in two-phase flow. New kernel functions for the rate of bubble break-up and coalescence are presented. The model is able to predict the bubble size distribution in bubble columns including the formation of large bubbles at high superficial gas velocities. Applying the concept of self-similarity an approximate analytical solution for the bubble size distribution can be found. With this solution the population balance is reduced to a transport equation for the average bubble volume. We implemented the transport equation in a commercial CFD-code to demonstrate the possibility of combining population balances with CFD. A dynamic two-phase flow model of the Eulerian-Eulerian type is used and calculations for three space coordinates are performed for cylindrical bubble columns.