In this thesis the transition to turbulence in pipe flow is investigated. At low Reynolds numbers, the flow returns to the laminar state spontaneously. At high Reynolds number a small perturbation causes the flow to suddenly become turbulent. In the intermediate regime localized turbulence is observed. This regime is investigated using experimental and numerical methods. It is shown that there is not a critical Reynolds number above which the turbulent state becomes an attractor, hence that it cannot return to the laminar state.