GSTR-1 is a monthly or quarterly return that registered taxpayers file under the Goods and Services Tax (GST) system in India. It contains details of all outward supplies of goods and services made during the reporting period. The return includes invoice-wise details of sales, exports, and other taxable supplies. Taxpayers must report details of Business-to-Business (B2B) and Business-to-Consumer (B2C) transactions. The return helps in reconciling input tax credit (ITC) for recipients of supplies. It is important for auto-populating data in GSTR-2A and GSTR-2B of buyers. Taxpayers with turnover up to ₹5 crore can opt for the Quarterly Return Monthly Payment (QRMP) scheme. Under the QRMP scheme, small taxpayers can file GSTR-1 every quarter instead of monthly. The due date for monthly filers is the 11th of the following month. Quarterly filers must submit GSTR-1 by the 13th of the month following the quarter. Non-filing of GSTR-1 can lead to penalties and blocking of e-way bill generation. Amendments to previously filed invoices can be made in subsequent returns. Details of debit and credit notes must also be reported in GSTR-1. Exports, deemed exports, and supplies to Special Economic Zones (SEZs) are recorded in this return. Nil GSTR-1 must be filed even if there are no sales in a given period. Taxpayers cannot file GSTR-1 if they have pending GSTR-3B for previous periods. The government uses GSTR-1 data to track tax compliance and detect mismatches. Auto-drafted GSTR-1 details are visible in GSTR-1A before final submission. Correct reporting in GSTR-1 ensures that buyers can claim their ITC without issues. Errors in GSTR-1 can be corrected in the next filing but not after the financial year ends. Late filing attracts a late fee of ₹50 per day for normal taxpayers and ₹20 per day for nil returns. E-commerce operators must report taxable supplies made through their platform in GSTR-1. Taxpayers must ensure that their invoice data is accurate before submission. The GST Network (GSTN) provides online tools and APIs for bulk invoice uploads. Large businesses use ERP software for automated GSTR-1 filing. The return does not include purchases, ITC claims, or tax payments, which are covered in GSTR-3B. Mismatches between GSTR-1 and GSTR-3B can trigger notices from GST authorities. GSTR-1 data is crucial for GST refunds on exports and inverted duty structure cases. Taxpayers can view their past GSTR-1 returns on the GST portal. Digital signature or e-signature is required for submission by certain categories of taxpayers. Accurate and timely filing of GSTR-1 helps businesses maintain smooth GST compliance.