A combined numerical and experimental study of a two-dimensional transitional separation bubble due to an adverse pressure gradient is reported. The experiments have been performed in the MTL wind tunnel with a contoured wall imposing an adverse pressure gradient on the flow over a flat plate. The separated shear-layer is highly unstable and transition to turbulence occurs in the flow. The experimental separation bubble flow is modelled numerically using two-dimensional direct numerical simulations (DNS). Prescribing free stream boundary conditions in the wall normal velocity the experimental bubble is reproduced. The development of artificially forced two-dimensional instability waves is investigated and good agreement is found between experiments, simulations and linear stability theory (LST). The performance of several engineering transition prediction methods applied on the present separation bubble is presented and compared. Methods based on simplifications of the e(n)-method yield predictions in accordance with experiments and DNS.