This document discusses the requirements for validly reopening an assessment under sections 147/148 of the Income Tax Act. It begins by outlining the objectives of presenting key cases and the Commissioner's power of review. It then explains that for a reopening to be valid, the Assessing Officer must have a 'reason to believe' that income was escaped within the 6 or 4 year time periods, depending on the amount. Several important cases are summarized that describe what constitutes a valid reason to believe. The document concludes by noting the judicial stance favors the assessee when all relevant information was disclosed originally, and warns the Commissioner to use review powers responsibly.