This thesis is a report of the transport properties of bilayer two-dimensional electron systems found in GaAs/AlGaAs double quantum well semiconductor heterostructures. When a strong perpendicular magnetic field is applied so that the total Landau filling factor is equal to one and if the two layers are close enough together, a novel quantum Hall (QH) state with strong interlayer correlations can form. This QH state is often described as an excitonic condensate, in which electrons...