Best judgment assessment empowers the assessing officer to determine income where returns or compliance are absent, after hearing. Section 271 provides for best judgment assessment where an assessee fails to furnish required returns or comply with specified notices or directions; the Assessing Officer may determine total income or loss and the payable sum to the best of his judgment after considering relevant materials. The officer must, subject to a narrow exception, serve a show-cause notice specifying date and time and give the assessee an opportunity of being heard before completing the assessment.