The motion of a single elongated (Taylor) bubble propagating in a transparent vertical pipe is studied experimentally in stagnant liquid, as well as in upward and downward liquid flow. Digital image processing of a sequence of video images serves as the main experimental method for the study of the Taylor bubble motion. In addition, the distribution of the velocities in front of the bubble and in the liquid film is measured using Particle Image Velocimetry. The relation between the Taylor bubble motion and the velocity field in front of it is discussed.