A closed model for slug flow pattern, which permits the prediction of all slug characteristics (for given gas and liquid rates) is presented. Aerated slugs (non-zero void fraction) and pure liquid slugs are considered. The model is based on new concepts of boundary-layer relaxation in a mixing region at the slug front and its recovery at the slug back. It is shown that for given gas and liquid rates, the model can, in principle, be applied for a wide quasi-steady frequency range in the developing region. However, stability analysis indicates that there exists a preferable (narrow) frequency range, associated with minimum pressure drop, where slug pattern stabilizes. Comparison with available experimental findings is satisfactory.