An investment declaration is a formal statement submitted by a salaried employee to their employer at the beginning of each financial year — typically in April or May — declaring their proposed tax-saving investments, eligible deductions, and exemptions for the year, so the employer can correctly compute and deduct TDS (Tax Deducted at Source) from the employee's monthly salary. The investment declaration covers all major tax benefit categories including: Section 80C investments (PPF, ELSS, EPF, life insurance premiums, NSC, home loan principal, tuition fees — up to ₹1.5 lakh), Section 80D health insurance premiums, HRA exemption details (rent paid and landlord's PAN), home loan interest deduction under Section 24(b), Leave Travel Allowance (LTA), and any other applicable deductions. Based on the declaration, the employer computes the employee's estimated annual income and tax liability, and deducts the appropriate monthly TDS so the annual liability is spread evenly. At the end of the year (typically between January and March), employees must submit actual investment proof documents — premium receipts, bank statements for FDs, ELSS investment certificates, rent receipts — to their employer for verification. Incorrect or insufficient investment declarations result in higher TDS deduction for the remaining months of the year or at year-end, creating cash flow inconvenience even though the actual tax liability remains unchanged.