|The spontaneous imbibition process in porous media is an important recovery mechanism and is governed by capillary forces that arise when two immiscible fluids are present in the pore space. This thesis presents new observations where the displacement process deviates from the widespread assumption of a uniformly shaped saturation front and the square root of time behavior proposed by Washburn. The influence of flow patterns during the onset period, a term used to describe the initial period of spontaneous imbibition, has been studied in different aspects of spontaneous imbibition, such as scaling of results from different systems and measurement of capillary pressure and wettability, in this experimental thesis. Co-current spontaneous imbibition experiments were performed on sand packed imbibition tubes applying a TEOFSI (Two-Ends-Open Free spontaneous imbibition) boundary condition, with one end face in contact with water and one in contact with oil. The spontaneous imbibition process was unaffected by any onset period for all imbibition tube experiments with a range of fluid viscosities and initial water saturations at strongly water-wet sand, except with an initial water saturation S_(w,i)=0.25 ±0.01. Access to local flow patterns during the onset period was achieved in three different porous systems: unconsolidated sands packed in glass tubes, epoxy-coated two-dimensional paper models, and cylindrical sandstone core plugs. A methodology was developed to compare experimental saturation development data with analytical solutions to investigate their assumptions and validity during the onset period. Two-dimensional paper models with a limited area open for imbibition showed that conventional one end open scaling groups are suitable in a limited case where the effect from the onset is suppressed due to the total duration of the spontaneous imbibition process. The saturation front in the models transformed as anticipated from the geometrical shaped of the samples. Positron emission tomography demonstrated the impact of non-uniform wettability in the epoxy-coated core plug, with long induction times and significantly deviating saturation fronts and development. A similar effect was observed using dyed non-wetting phase, to enhance identification of advancing displacement fronts in the packed sand columns. The dye changed wettability locally and resulted in irregular saturation development.