Polyorthoesters have been the most intensively studied polymers for biomedical applications for their low immunogenicity, good biocompatibility and excellent mechanical properties. In the last decade, the synthesis of polyester-polyether block copolymers has attracted increasing attention, because they may be used in several pharmaceutical and medical applications such as sutures, implants for bone fixation, carriers in drug delivery and temporary matrix or scaffolds in tissue engineering. This book includes the last two decades when block copolymers containing PEO, hydrophilic B-blocks, and PAA or PAGA, hydrophobic A-blocks, have been widely utilised to formulate parenteral drug delivery systems for peptides and proteins, such as micro and nanoparticles, implants, micelles, hydrogels and scaffolds.