Pigs (pipeline inspection gauges) are free moving piston-like devices, that are used for inspecting or cleaning pipelines in for example the oil and gas industry. Here they are used for onshore and offshore pipelines, where they are propelled by the production fluids. A better control of the velocity of the pig in the pipeline can be achieved by using so-called bypass pigs, which have an opening (bypass) that allows that some fluid passes by which reduces the pig velocity. A new development is a by-pass pig with speed control, in which the bypass area is adjusted when the pig moves through the pipeline to obtain a constant preset pig velocity. The goal of this research was to accurately describe the forces on a pig, with special emphasis on the friction between the pig and the pipe wall. Such a wall friction model can be used in pipeline simulation tools, that are used to prepare a pigging operation. The friction force is divided into two parts: (1) the normal force that the pig imposes on the pipeline wall (F_wall), and (2) the coefficient of friction (?) which converts F_wall to a friction force. F_wall is calculated by a non-linear finite element model for a 12" sealing disk for a range of oversizes. A uniaxial tensile lab test was performed to fit the material coefficients used in the non-linear Mooney-Rivlin material model. It was shown that the results could be matched by a linear elastic analytical model with a correction factor Cor = 1.45. An experiment was designed to measure both ? and F_wall for a range of pig disk oversizes. These values were measured for sealing disks with different thicknesses and made of different materials. The models were able to accurately describe the experiment results. Due to lubrication the coefficient of friction can drop by orders of magnitude. Lubrication can be described by the Reynolds equation for the fluid pressure of thin fluid films. The material of a pig that makes contact with the pipeline wall is deformed by this fluid pressure which can be described by soft elastohydrodynamic lubrication theory. To couple the linear elastic material of the sealing disk to the non-linear Reynolds equation, two numerical approaches were followed. Firstly, the linear elastic contact was described by a row of closely spaced linear springs. Secondly a lubrication model was made in which the linear elastic contact was described by the boundary element method. It was shown that these models give a good qualitative description of the lubrication behaviour. The model validation is left for future work. In addition, pressure drop measurements were performed in a flow loop for a model of the bypass pig. These measurements were compared with correlations from the literature.