PEGylation technology and key applications are introduced by this topical volume. Basic physical and chemical properties of PEG as basis for altering/improving in vivo behaviour of PEG-conjugates such as increased stability, improved PK/PD, and decreased immunogenicity, are discussed. Furthermore, chemical and enzymatic strategies for the coupling and the conjugate characterization are reported. Following chapters describe approved and marketed PEG-proteins and PEG-oligonucleotides as well as conjugates in various stages of clinical development. The volume closes with chapters on FDA regulations and EMEA guidelines for these drugs and general perspectives for future developments.