Note: [Deprecated Content] This was written while ASP.NET and .NET Core was in beta (around beta5 or beta6). Since then a lot of stuffs have changed and moved around. Stuffs described below may or may not be true for current stable releases of ASP.NET Core. I recommend not to follow along and get confused. Use official configuration documentation that explains it correctly - ASP.NET 5 removed the web.config XML based configuration files and bring JSON based project.json and other similar files to maintain configuration related to your application. project.json is the main file that contains meta-information / configuration for running your ASP.NET 5 application. This JSON file consist of a node named compilationOptions which defines a child called languageVersion. Currently ASP.NET 5 only supports C# language. languageVersion can be one of the following options: It provides way to choose which » Read more