Theoretical guidelines are established for designing miscible blends of amorphous polyolefin copolymers. On the basis of calculations for an athermal and incompressible model of copolymer melts, limits are placed on the compositions and structural differences between blend components that are consistent with thermodynamic stability of a single liquid phase. Specific cases analyzed include binary blends of random copolymers containing short branches and blends of graft polymers with long flexible branches, either periodically or randomly placed. The predictions are shown to be in good agreement with recent experimental studies of miscibility in model polyolefin copolymer blends.