When you try to launch BTS (or Warlords or Colonization) on OS X Yosemite, it fails with the message "Failed Initializing Python". This is because Yosemite drops support for Python 2.3, which is needed by Civ4 and it's expansions. The solution is to reinstall Python 2.3 on Yosemite. Unfortunately I couldn't find an OS X version of 2.3 on Python.org, so instead I copied the relevant folders across from Mavericks. This fix involves making some additions to folders and frameworks located in OS X's System and Library folders. It shouldn't cause any problems, but please be extremely careful and be sure to have your system backed up before trying this. Just in case. I've included Python 2.3 and 2.5. Both were present in Mavericks but gone from Yosemite. I don't know if both are needed by BTS, but I thought it best to include both just in case. I had to use 7zip to pack the files so they be small enough to host on with my sharing service. If you don't already have a way to unpack 7zip I highly recommend getting 'The Unarchiver' from the MacAppStore. It's free and, in my opinion, an essential Mac utility. Step 1: Download the necessary files from here. Unzip them. You should have a folder called 'BTS Yosemite Python Fix'. These are taken direct from my installation of 10.9.4 and you may copy them from Mavericks yourself if you prefer. Locations are described below. Step 2: Navigate to <Your Drive>/Library/Python/. This is NOT the Library folder in your Home folder. In this location you should see two folders called '2.6' and '2.7'. From the 'BTS Yosemite Python Fix' folder, copy the folders '2.3' and '2.5' to this location. You'll need to enter your administration password to do this. Step 3: Navigate to <Your Drive>/System/Library/Frameworks/Python.framework/Versions/. There should be 2 folders and 3 aliases here. You need to remove the aliases '2.3' and '2.5'. From the 'BTS Yosemite Python Fix', open the 'Framework' folder. You should see another '2.3' folder and another '2.5' folder. Copy these to where the aliases used to be. Again, your administration password is required. Don't copy the 'Framework' folder itself. BTS (and Warlords and Colonization) should now work. Vanilla Civ requires additional steps (see post #8) as it's blocked by OS X for being too ancient.