At a total filling factor of one two closely-spaced two-dimensional electron systems (2DES) can be viewed as a Bose-Einstein condensate of excitons, where electrons from one 2DES couple to vacant states from the other 2DES. The pairing of electrons and holes (i.e., excitons) is mediated by Coulomb interactions. In this thesis this excitonic state was investigated by interlayer tunneling and Coulomb drag experiments. The Coulomb drag experiments on Corbino ring structures have shown a minimum in the conductivity, implying an energy gap at the Fermi energy due to exciton condensation. The tunneling experiments revealed a critical behavior and an I/V curve reminiscent of the Josephson effect of superconductivity.