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.