The effects of boundary conditions, most importantly sidewall friction, on the outcome and subsequent interpretation and analysis of physical models in geotechnics are seldom given the attention required. Sidewall friction has significant influence on stress conditions within a specimen or model, and as such will affect the distribution of strain within the soil mass. A set of tests with fine grained uniform sand was performed in which sidewall friction and sidewall fixities were controlled and measured as loads were applied to the soil mass. As the specimen was loaded, shear stresses at the boundary of the test box, over the height of the specimen, were measured. Analysis of the test data reveals that during the loading stage of the experiment the development of shear stress is uniform and well understood. During unloading, the situation was shown to be more complex; sidewall shear is nonuniform over the sidewall height and changes sign as unloading progresses. The experimental data and analysis illustrate that by applying friction reduction measures, the effects of sidewall friction are significantly reduced. It is suggested that friction reduction measures are almost mandatory in two-dimensional physical model and element testing where the model thickness is typically very small. This could also be true in axisymmetric conditions, as illustrated by results from model cone penetration testing.