Long liquid slugs reaching several hundreds pipe diameter may appear when transporting gas and liquid in horizontal and near horizontal pipes. The long slugs cause system vibration and separation difficulties that may lead to operational failures. Identifying and predicting the time and length scales of slugging is important for gas and oil production technologies (e.g. for the design of offshore gas and oil pipelines and process facilities). Although mainly short hydrodynamic slugs (40 pipe diameters) have been observed in offshore production fields, the appearance of the long slugs becomes more likely as the field becomes older the operation pressure drops. Therefore, predicting the transition between the different slug types and the flow conditions at which the long slugs appear may be crucial preventing or reducing the negative effects of slugging. The approach adopted in this study is the construction of simplified theoretical models that successively approximate the flow conditions and the corresponding time and length scales of slugging. Experiments and numerical modelling have been performed for validation and comparison matters. The first part of the research deals with identifying the long slug region and sub–regions. Experiments carried out by Zoeteweij (2007) present a detailed flow map for the long slug region and the transition to hydrodynamic slugs or stratified wavy flow. For the prediction of the long slug region a simplified predictive model was constructed. The model calculates the average slug length based on the volumetric liquid rates adjoining the slug, and derives the change in the liquid level, at the tail of the slug, by linear kinematic relation between the tail and the following upstream wave. The model predicts the transition from hydrodynamic to long slugs with a satisfactory agreement. In the second part of the research the emphasis is put on predicting the transition from stratified flow to slug flow or roll–waves. Slugs formed by coalescence between roll–waves are hydrodynamic. Hence, only the flow conditions that lead to a direct transition from strat ified flow to slug flow (i.e. not via roll–waves) may lead to long slugs. For the prediction of the transition to slug flow or roll–waves a theoretical model was developed. The model tracks the displacements of the crest of a long wavelength wave in axial and radial directions. If the wave crest reaches the top of the pipe a slug is formed, whereas if it approaches the downstream end of the wave a roll–wave is produced. Besides to the predictive tool provided, the model sheds some light on the stage prior to forming a slug. The third part of the research considers the effect of the operation pressure on the slug length, and the effect of the liquid excess between the slug front and tail at the formation time. Measurements by Kristiansen (2004) for two–phase air–oil and SF6 gas–oilwere investigated. The measurements were carried out at P = 1–8 barA with high density SF6 gas simulating a pressure up to 65 bar. Three different types of slugs were categorized based on the liquid excess. Slugs with larger liquid excess at formation can grow to become longer. Even a small difference in the liquid excess may lead to a large difference in the slug length. However, at high operating pressures there is no liquid excess and only hydrodynamic slugs are observed. In the final part of the research we investigated and derived the slug frequency by the frequency of vortices due to turbulence in the gas and liquid. We found that the slug frequency and the frequency of oscillation at the interface behave similarly to the frequency of oscillations in the gas phase. However, the intensity of the oscillation at the interface is dominated by the liquid phase. The proposed mechanism for the formation of slugs covers a large range of pipe diameters and flow conditions. Moreover, it reveals the significance of the small–scale initial turbulence on the final development of the large–scale slug flow.