Abstract: Drug-drug complexes play important roles in improving the physicochemical properties of drugs including the solubility, dissolution rate and stability of the active pharmaceutical ingredients (APIs). In this paper, the design, synthesis, characterization, changes in physicochemical and pharmacologic properties, structural polymorphisms and the research and development pipelines of a variety of drug-drug cocrystals/salts synthesized based on the crystal engineering design are reviewed. This may provide theoretical support for the development of the new solid-state combinational drugs.