The flow patterns of pneumatically generated foams flowing in vertical pipes and their associated structure, pressure drop and liquid entrainment characteristics have been investigated. Experimental results have been obtained on the effects of liquid properties, surfactant type and concentration, foam generator, and pipe diameter. At low flowrates, the foam has a dry cellular texture with polyhedral cells and flow occurs entirely by slip near the wall. At high flowrates, a wet foam with spherical bubbles is formed and how is characterised by strong bubble mobility and intensive bubble recirculation. Flow was characterised by a constant pressure gradient under both flow regimes, and foam rheology was successfully described by a power-law model. Friction factors were determined for all systems studied and the data lied remarkably on a unique line on the friction factor-Reynolds number plot. An explicit relationship for predicting the friction factor in foam flows is presented, based on a wide range of experimental conditions. The model has practical significance in that pressure drop in foam flow can be calculated using a constant friction factor along a pipe of constant cross section, in any flow regime.