Maybe they disappeared in a move. Or your baby sister tore them to shreds. Or the dog ate them. For whatever reason, most of us have, at some time, lost building instructions. Apart from trying to put the thing back together from memory, stealing your best friend’s instructions, or just plain never building the set again, there are a number of sources online for you to recover instructions.
The first and best source is the LEGO website itself. Down at the bottom of each page in their standard footer, there is a link for Customer Service. That page has a couple links on it that say “Building Instructions.” There you can enter the set number, a key word, or search by a set brand (i.e. castle, harry potter, etc.) While they don’t have all the sets ever made, they do have all the recent ones. Once you’ve entered a set or a keyword you’ll get a list of all the applicable sets in their database. Click on one to see a thumbnail image of the set. Then, once you’ve found your set, you can download the instructions as a pretty good quality PDF.
Another place to find scanned images of most of the sets ever made is at The Brickfactory. Keep in mind that these won’t be nifty PDFs you can download. Each scan is a big JPG file sitting on a web-page. If you’re concerned about the legality of saving and/or printing these scans, The Brickfactory has respectfully posted a disclaimer that links to LEGO’s official “fair play” standards. There is a paragraph at the bottom of the page entitled, “Scanning Of Copyrighted Materials Into A Web Site” where the LEGO company specifically addresses this sort of site and essentially says, “hey, we own the copyrights, but at present we are not going to make a stink about people posting old building instructions on the internet if they’re not trying to make money off it, or make us look bad.” (That was my paraphrase, but please do go ahead and read the page for yourself.)
Finally, If you want a real, live set of paper instructions, Bricklink is a good place to go to find people selling building instructions. From the Reference page, use the Item Types drop-down menu to select “Instructions.”
Hope that helps. If you have found other helpful ways to track down building instructions, feel free to post a comment, and I may incorporate it back into the article at some point.