PEGylation is the covalent attachment of protein and/or peptide to poly(ethylene glycol). The poly(ethylene glycol) is a polymer, non toxic, non immunogenic, and FDA (Food and Drug Administration, USA) approved. In the last years, PEGylation has been used to improve the physicochemical properties of some proteins and therapeutic drugs; this technology has impacted heavily on the bio-pharmaceutical industry. PEGylation prolongs the body-residence time and stability, decreases the proteolysis and renal excretion. Since the emergence of this technology, some proteins have been PEGylated for the treatment of diseases including hepatitis C, leukemia, rheumatoid arthritis, etc. This review presents a description of the PEGylation development in the last years and the chemical procedures used to obtain some bio-conjugated products. Strategies of purification used to obtain PEGylated proteins are reviewed; purification is one of the major problems to establish suitable processes due to the fact that the reaction can generate bioconjugates with different degree of PEGylation. Finally the applications of PEGylated proteins and the future challenges that are identied for generic application are presented.