Hi, Using Visual Studio 2017 15.5, the following minimal test case causes a linker error with Boost 1.66 RC1 but builds fine with 1.65.1: Produces... 1>------ Build started: Project: aZ80, Configuration: Debug Win32 ------ 1>libboost_timer-vc141-mt-gd-x32-1_66.lib(cpu_timer.obj) : error LNK2005: "public: static class boost::chrono::time_point<class boost::chrono::steady_clock,class boost::chrono::duration<__int64,class boost::ratio<1,1000000000> > > __cdecl boost::chrono::steady_clock::now(void)" (?now@steady_clock@chrono@boost@@SA?AV?$time_point@Vsteady_clock@chrono@boost@@V?$duration@_JV?$ratio@$00$0DLJKMKAA@@boost@@@23@@23@XZ) already defined in libboost_chrono-vc141-mt-gd-x32-1_66.lib(chrono.obj) 1>libboost_timer-vc141-mt-gd-x32-1_66.lib(cpu_timer.obj) : error LNK2005: "double __cdecl boost::chrono::chrono_detail::get_nanosecs_per_tic(void)" (?get_nanosecs_per_tic@chrono_detail@chrono@boost@@YANXZ) already defined in libboost_chrono-vc141-mt-gd-x32-1_66.lib(chrono.obj) 1>C:\Projects\aZ80\vs\Debug\aZ80.exe : fatal error LNK1169: one or more multiply defined symbols found Swapping the order of the includes appears to workaround the problem. Mark. -- Sent from: http://boost.2283326.n4.nabble.com/Boost-Dev-f2600599.html