How to diff remote files using ssh ?

One file is on remote server

diff /path/to/file <(ssh RemoteServer 'cat /path/to/file')

You can also use vimdiff hack (do not forget double slash !!!)

vimdiff /path/to/file scp://remotehost//path/to/file

Both files are on remote servers

diff <(ssh RemoteServer1 'cat /path/to/file') <(ssh RemoteServer2 'cat /path/to/file')