We show that three‐dimensional arrays of electrically small, anisotropic particles are excellent media for wave‐mixing processes at microwave and millimeter wavelengths. Electrostrictive torques rotate the particles in such a way as to form orientational index gratings that can control the attributes of microwave or millimeter wave radiation. Theory asserts that this medium is highly favorable for phase conjugation via quasidegenerate four‐wave mixing at these wavelengths.