Liquid-liquid phase separation (LLPS) of biomacromolecules such as proteins and nucleic acids has been widely found in the cytosol, forming coacervates, a.k.a. membraneless organelles, which compartmentalize the cells into different functional and structural regions. Interestingly, we found that low-molecular-weight compounds may form coacervates via LLPS in the aqueous solution. Surprisingly, some coacervates (we call molecular coacervates) may encapsulate and deliver proteins and nucleic acids across the plasma membrane into the cell. These unique features have inspired our team to explore the use of coacervates as delivery vehicles or transfection agents for proteins and nucleic acids. We then developed a range of coacervate-based technologies to treat diseases, with a focus on cancer immunotherapies.