The first edition of the EPA Peer Review Handbook was issued in 1998 and was intended to serve as a single, centralized source of implementation guidance on peer review for EPA staff and managers. Subsequent revisions of the Handbook have added necessary clarifications, incorporated insights and experiences gained through its use, and integrated changes to reflect updated government-wide guidance or policy related to peer review. These revisions have increased the transparency and accountability of peer review and helped ensure that Agency decisions are based on sound and defensible science. For the 4th edition, the EPA's STPC determined that revisions were needed to incorporate several recent EPA policy and process changes related to peer review. Although the 4th edition draws heavily from the 3rd edition, it has been reorganized to emphasize the elements and tools needed to implement a systematic peer review. It retains, however, the "question and answer" format throughout. New flowcharts and checklists have been added, and several substantial updates are included, such as the additional guidance on appearance of a loss of impartiality in external peer reviews, new information on organizational changes and oversight responsibilities, and changes related to the issuance of recent policies and procedures associated with the EPA's Information Quality Guidelines (IQG). The 4th edition also describes process changes for contractor-managed panel peer reviews of scientific and technical documents designated as Influential Scientific Information (lSI). The process is intended to reduce the potential for organizational or personal conflict-of-interest (COl) concerns. Early public participation in the nomination and selection of peer reviewers and increased internal oversight are features of the process.