Polyethylenimine “Max” (PEI MAX) is a powerful, trusted, and cost-effective reagent widely considered as a current gold standard for both in vitro and in vivo transfection. PEI MAX has a high density of protonatable amino groups, with amino nitrogen as every third atom. This imparts a high buffering ability at nearly any pH. Hence, once inside the endosome, PEI MAX disrupts the vacuole and releases the genetic material into the cytoplasm. Stable complexation with DNA, efficient entry into the cell, and ability to escape the endosome makes PEI MAX a highly efficient transfection reagent which is compatible for a wide range of cell lines/types including the most commonly used HEK293 and CHO cells grown in adherent and suspension cultures. PEI MAX is capable of yielding high efficiency cell lines without compromising cell viability compared to other PEI and liposomal transfection reagents available in the market.