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This dissertation describes biological interactions of synthetic polymers relevant to two major biomedical applications, gene delivery and targeted drug delivery. The first part describes biological interactions of polycationic polymers that have been commonly used as gene delivery/cell transfection agents. This study reveals that polycationic polymers such as amine terminated poly(amidoamine) (PAMAM) dendrimers, poly-L-lysine, polyethylenimine, diethylaminoethyl-dextran induce membrane permeabilization in living cells. Exposure of cells to the polycationic polymers caused enzyme leakage out of the cells, polymer internalization into the cells, and diffusion of small molecular probes in and out of the cells. In contrast, charge neutral acetamide and negatively charged carboxylate terminated PAMAM dendrimers, as well as polyethyleneglycol and poly(vinyl alcohol) do not enter the cells or cause permeability of the cell membranes. By combining our previous AFM observation and a variety of in vitro tests presented here, we conclude that a nanoscale hole formation mechanism is an important pathway for polycationic polymer-cell interactions. The second part of this study deals with multivalent interaction of cancer cell targeting dendritic nanodevices with a receptor protein target. Dendrimer-based anti-cancer nanotherapeutics containing ∼5 folate molecules have shown in vitro and in vivo efficacy in cancer cell targeting. Multivalent interactions have been inferred from observed targeting efficacy but have not been experimentally proven. This study provides quantitative and systematic evidence for multivalent interactions between these nanodevices and folate binding protein (FBP). A series of the nanodevices were synthesized by conjugation with different amounts of folate. Dissociation constants ( KD) between the nanodevices and FBP measured by SPR are dramatically enhanced through multivalency (∼2,500-170,000 fold). Qualitative evidence is also provided for a multivalent targeting effect to KB cells using flow cytometry. These data support the hypothesis that multivalent enhancement of KD, not an enhanced rate of endocytosis, is the key factor resulting in the improved biological targeting by these drug delivery platforms, providing a design guide for future receptor targeting agents.