The delivery of biologicals across endosomal membranes by pH responsive polymers The efficient and safe delivery of complex biological agents offers great potential for the challenges of our time. Polymers offer considerable advantages in this regard not only because of their low risk for immunogenic reactions their comparatively inexpensive large scale production and their long shelf life. The biological properties of organisms organs tissues and cells determine the design of safe and efficient delivery systems and should be considered as inspiration for new attempts to create effective materials. In addition to the balance between hydrophobicity and hydrophilicity the degree of charge of a nanocarrier is also critical for efficient delivery and overcoming the endosome. Cationic charges do not only enhance cellular uptake but can also mediate a crossing of the endosomal barrier due to membrane interactions. The introduction of hydrophobic or even lipophilic components can further significantly increase membrane interactions. Besides the interaction with the cell membrane it is important to consider that both the degree of charge and the hydrophobic hydrophilic balance also affect the packaging and protection of sensitive biological cargo such as genetic material. However cationic charges and hydrophobic components introduce some disadvantages as electrostatic and hydrophobic interactions may disrupt the integrity of other cellular membranes triggering toxic effects. Fine adjustment of the degree of charge and the hydrophobic hydrophilic balance offer the possibility of balancing the requirements for a safe yet efficient delivery process. In the present dissertation various polymer systems with adjustable charge density and hydrophobic hydrophilic balance were investigated with respect to their delivery efficiencies. A particular focus was set on the key step in delivery the endosomal release.