mTM-align is a server for efficient protein structure comparisons, which includes two related modules. The first is for fast database search with one input structure. The second is for multiple structure alignment with two or more input structures. For the first module, it takes about 2-5 seconds to search for similar structure proteins. After the search is done, the users are able to select other structures from the returned list to perform multiple structure alignment. For the second module, it takes a few seconds for input of ~10 structures of medium size. The server is free for all users.