Gotta read the whole thing: "For the Leopard-from-Tiger direction, you need a separate, free application. Screen Sharing is based on, and compatible with, VNC, a widely used remote-control protocol. You can thus use a VNC client under Tiger to connect to Leopard systems. First, on the Leopard Mac, in the Sharing preference pane's Screen Sharing item, click Computer Settings, and then check the VNC box and enter a password; note that VNC doesn't rely on or integrate with Mac OS X user accounts. Back on the Tiger Mac, install the free Chicken of the VNC, and use it to connect to and control the Leopard Mac. (A VNC client on Macs running older versions of Mac OS X or computers running other platforms can also work with Leopard's Screen Sharing.) Chicken of the VNC can discover local systems, including those running Leopard, that are sharing screens by using Bonjour; or you can enter a remote, routable IP address. Screen Sharing plus NAT traversal simplifies having remote access to your own system or systems, as well as providing tech support to colleagues and your family members."