SCENARIO: I have a network of Macs. On my main machine (a Mac Pro), I have set up bookmarks in Safari that link to the Screen Sharing URLs for other Macs on my network (Mac minis, MacBook Air). For example: vnc://192.168.1.51 vnc://192.168.1.52 So I can kick off a Screen Sharing session with one of those Macs simply by clicking the appropriate bookmark in Safari. PROBLEM: Sometimes when I do this, one of two things happens... either the Screen Sharing program launches in the dock but the window for the other Mac does not appear at all, or the Screen Sharing program launches and a small black window (with nothing visible inside) appears. Killing and restarting Screen Sharing does not help, and restarting the remote Mac does not help. SOLUTION: One way to solve this problem is to restart the Mac where you are attempting to use Screen Sharing. But this is overkill; it works because it incorporates the second, easier solution to the problem: 1. Launch the Activity Monitor program. 2. In the Process Name list, look for "NetAuthAgent". It may show as "Not Responding". 3. Select NetAuthAgent, and then click Quit Process (the red stop sign button at the top of the Activity Monitor window). Then, click Force Quit. Your Screen Sharing should now work. It appears that as part of connecting to the remote Mac, your Mac must authenticate itself... and NetAuthAgent is part of this process. If this program freezes, hangs, or locks for any reason, the authentication will not work, and you cannot use Screen Sharing. Killing NetAuthAgent is harmless, as far as I can tell, and it will automatically relaunch the next time it is needed. For anyone else having problems with Screen Sharing not working correctly, I hope this information helps.