Many organic semiconductor structures display strong magnetoelectrical and magneto-optical effects at rather weak magnetic fields. In this chapter we develop a simple model for the magnetic field dependence of luminescence emanating from donor/acceptor molecule bilayer structures. The physical mechanism underlying the magnetic field effect is attributed to a competition between the applied magnetic field and small random internal fields associated with the hyperfine interaction, the spin-orbit coupling, and g-factor differences for electrons and holes forming weakly interacting pairs of different total spin. These random fields enable spin relaxation between the different spin states of the pairs. We explore possible carrier injection scenarios, polarized and unpolarized, and we discuss the different line shapes that may result from different time-correlation functions describing the random fields. Lastly, we argue that the formalism discussed lends itself also to the description of magneto-electrical phenomena that have been observed recently in the tunneling characteristics of self-assembled monolayers of organic molecules.