Mortgage underwriting is a detailed review of a borrower's financial information, credit history, income, employment, assets, and the home's value to determine mortgage eligibility. It typically takes days to weeks and involves verifying capacity to repay, creditworthiness, collateral value, and conditions like debt-to-income ratio. Denials often result from incomplete info, credit changes, or unmet qualifications. Approval requires satisfactory appraisal, title search, and financial review.