The release candidates for the 1.66.0 beta release are now available at: https://dl.bintray.com/boostorg/beta/1.66.0.beta.1/source/ You need to scroll past the first release candidates to find the downloads. The SHA256 checksums are as follows: e7b23e638e8c3a9da1512439385b04850829cea7bd56d8398677c096857d830b boost_1_66_0_b1_rc2.7z 5d0405eb73bd609ebcd54bbc96d4f1e2d97c51b96c4703706695ddd94f942d6b boost_1_66_0_b1_rc2.tar.bz2 f0f455e39bc47b2b670a17cb221687506e9a22dc478956d3d8aabb9cb835a7dc boost_1_66_0_b1_rc2.tar.gz 694c8b9e902b7981670bce166f675f3875c19326936db51aabd98d2ddfea9b45 boost_1_66_0_b1_rc2.zip The in progress release notes are at: http://www.boost.org/users/history/in_progress.html And the documentation is at: http://www.boost.org/doc/libs/1_66_0_beta1/ As always, the release managers would appreciate it if you download the candidate of your choice and give building it a try. Please report both success and failure, and anything else that is noteworthy.