Assessment under Section 147 permits reassessment when income has escaped, subject to notice and time-limit exceptions. Assessment under Section 147 permits an assessing officer to assess or reassess income believed to have escaped assessment and to recompute losses or allowances; additional income discovered during those proceedings may be included. A four-year limitation for action under Sections 143(3) or 147 applies except where the assessee defaulted in filing or disclosure, while no time limit applies if the assessee holds any asset or financial interest outside India. Reassessment is subject to notice requirements under Section 148 and the procedure in Section 148A, and does not extend to matters already under appeal, reference or revision.