The resonant interaction of a triad of inertial waves is investigated theoretically. An additional condition for resonance is found to be necessary compared to short internal wave interactions—namely, that the polarization must be specified. One of the quantities conserved is&ohgr;· ucalculated for each wave separately and summed over the triad. Here,&ohgr;is the vorticity field anduis the velocity field of the inertial wave. Resonance diagrams are given and two explicit examples of amplitude variations are calculated.