Polymers comprise the second largest group of nucleic acid delivery vehicles after lipids. Cationic polymers form stable complexes (polyplexes) with anionic nucleic acids and offer a versatile, scalable, and easily adjustable platform for efficient nucleic acid delivery while minimizing immune responses and cellular toxicity. Linear cationic polymers are the most widely studied polymeric nucleic acid carriers. 300 PEI, poly(L-lysine) (PLL), poly (2-(dimethylamino)ethyl methacrylate) (PDMAEMA), poly 2-aminoethyl methacrylamide (PAEMA), poly(amidoamines) (PAAs), and poly(β-amino esters) (PBAEs) have been studied for drug delivery (Figure ). These linear polymers differ in their cation types (primary amines such as PLL, secondary amines such as PEI, or tertiary amines such as PDMAEMA), and their cationic charge density.