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Look for the set number on the side of the box or on the instructions. If you can’t find the box or instructions, you can look up the product number online.  Go to www.lego.com/en-us/products. Scroll through the pictures of Lego products and find your theme and set. Copy the set number so you can use it later when you are requesting parts. If you have damaged or missing LEGO pieces you can request a replacement part to be shipped for free. Access the customer service portion of the LEGO.com homepage by clicking on the link at the bottom of the page.  Click on the “Missing Parts” link. Enter your age and your country of origin on the “Bricks & Pieces” page (You must be 18 years old or older to shop online at the LEGO.com website). Click “Next.” Ask a parent for help if you are under 18. ” Type in the set number. Click “Go.” This will bring you to a listing of all the parts contained in the set number you entered.  Not all parts are available for replacement at all times. If your part is out of stock, you can request a new one be sent once it is available. Click “Checkout” after you are done choosing pieces. Click “Next” and then confirm the shipment. Unless you chose to buy pieces, the order should be free of charge. Your LEGO should arrive within a couple of weeks.

Summary:
Find the box for your LEGO set that is missing parts. Go to LEGO.com. Select “My new set has a missing piece” or “My new set has a broken piece. Select the parts that are missing. Enter your shipping address, including your email address. Look for your email confirmation.