I upgraded to XCode 4 recently which only includes/supports the 10.6 SDK. While trying to build Boost 1.46.1 with a deployment target (macosx-version-min) of 10.4 I get a bjam error about 10.4 not being a valid option for macosx-version-min (the only valid option listed is 10.6). Perhaps I'm confused about something but I was under the impression that you could build using a newer base SDK than your target version. With XCode 3 I just used 10.4 as the base SDK and deployment target as that SDK is included and had no issues. I built wxWidgets fine using a 10.4 deployment target and 10.6 base SDK. Does anyone have experience with doing this in Boost? Regards, Brad Anderson