Pegylation, the technology of polyethylene glycol (PEG) conjugation, holds significant promise in maintaining effective plasma concentrations of systemically administered drugs that might otherwise be hampered in vivo by a number of factors, such as rapid elimination by the kidneys. Mobile, nontoxic PEG chains can be conjugated to biotherapeutics, increasing their hydrodynamic volume, which can in turn prolong their plasma retention time, increase their solubility, and shield antigenic determinants on the drug from detection by the immune system. Attaching PEG molecules for optimal pharmacokinetics without obstructing the active sites that are essential for drug efficacy is a major challenge in pegylation. Current pegylation technology uses linkerless conjugation methods to produce coupling without added toxicity or immunogenicity, and may keep the innate surface charge of the pegylated molecule intact. In addition to controlling the size and complexity of PEG molecules, the attachment site can be manipulated to avoid steric hindrance of the drug's active receptor-recognition or substrate-interaction site. A few pegylated drugs have been engineered to have an improved pharmacokinetic profile with preserved bioactivity. They often have prolonged steady plasma concentrations in vivo, thereby making a reduced number of doses possible. Other interesting effects have also emerged, such as the self-regulating pharmacokinetics of pegfilgrastim, a pegylated version of the granulocyte colony-stimulating factor filgrastim that is administered for management of chemotherapy-induced neutropenia. The improved dosing schedule, with longer intervals between administrations of the pegylated agents, will improve compliance and quality of life in patients with chronic disease.