A comprehensive one-dimensional fully coupled hydrodynamic treatment of the film profiles behind liquid slugs in horizontal and inclined gas-liquid slug flow is presented Experiments were performed in horizontal air-water pipes 32 and 50 mm in diameter, the smaller also being inclined to an angle of 5 degrees. Time traces of film thickness were produced with the use of a wire-probe technique. The experimental results are compared against the theory presented here, as well as previously reported hydrodynamic models. It is concluded that the film profiles calculated by the use of one-dimensional models are highly sensitive to the choice of input variables, for example, the slug translational velocity; however, good agreement is achievable provided that consistent hydrodynamic equations and correct input variables are used. It is hoped that the work presented here will influence future mechanistic models of slug flow.