How to See the Light Cone

New experiments which probe the behavior of products of electromagnetic currents at short and lightlike distances are investigated. They require the detection of a massive $\mu$ pair in electron-positron annihilation or in electroproduction. The former reaction is studied in detail and the (considerable) background is discussed. It is shown that, in suitable kinematic regions, one can approach both the Bjorken-Johnson-Low limit and the scaling limit, and put to severe test many recent ideas regarding products of currents. In the Bjorken-Johnson-Low limit one can test models of equal-time commutation relations and measure the spectral functions of an axial-vector current. In the scaling region one can measure the matrix elements of the bilocal operators appearing in the light-cone expansion of the product of electromagnetic currents, and test the tensorial structure of this expansion which emerges from quark models. Finally, in the inclusive reaction, where a massive $\mu$ pair is produced in addition to any number of hadrons, the large hadronic mass limit of the cross section is completely determined in terms of a commutator of bilocal operators.