Adenoviral gene therapy vectors have been widely studied and used. Their extremely high transduction efficiency and gene delivery in vivo make them attractive for cancer gene therapy approaches. While they are robust, they are also very immunogenic. One approach to mitigate the immunogenicity of adenoviruses and to evade neutralizing antibodies is to coat the virus with the hydrophilic polymer polyethylene glycol (PEG) (1). This chapter details the steps involved when going from recombinant adenoviral vector plasmid all the way to validated PEGylated adenovirus product.