You need to add a new framework configuration which (still?) doesn’t come by default with the download of NAnt. Insert the following into your NAnt.exe.config file (alongside the similar looking framework configurations): <framework  name=”net-4.0″   family=”net”   version=”4.0″   vendor=”Microsoft”   description=”Microsoft .NET Framework 4.0″   sdkdirectory=”${path::combine(sdkInstallRoot, ‘bin’)}”   frameworkdirectory=”${path::combine(installRoot, ‘v4.0.30319’)}”   frameworkassemblydirectory=”${path::combine(installRoot, ‘v4.0.30319’)}”   clrversion=”4.0.30319″>   <runtime>     <probing-paths>       <directory name=”lib/net/2.0″ />       <directory name=”lib/net/neutral” />       <directory name=”lib/common/2.0″ />       <directory name=”lib/common/neutral” />     </probing-paths>     <modes>…