Medical expense deduction under Section 80DDB reduces taxable income for specified disease treatment when actually incurred and documented. A resident individual or HUF may claim a deduction under Section 80DDB for specified chronic disease medical expenses equal to the lesser of actual expenditure and the prescribed ceiling, with a higher ceiling for senior citizens; claimants must provide a specialist's certificate, the deduction is reduced by insurance or employer reimbursement, applies only to expenses actually incurred for the individual or wholly/mainly dependent relatives, and is distinct from health insurance premium benefits under Section 80D.