If you are using both ASP.NET MVC and ASP.NET Web API in your application, you need to add the Abp.Owin NuGet package to your project.

Installation

Add the Abp.Owin NuGet package to your host project (generally, to the Web project).

Install-Package Abp.Owin

Usage

Then call the UseAbp() extension method in your OWIN Startup file as shown below:

[assembly: OwinStartup(typeof(Startup))]
public class Startup
{
    public void Configuration(IAppBuilder app)
    {
        app.UseAbp();

        //your other configuration...
    }
}

If you are only using OWIN (say, in a self hosted Web API project), you can use the override of UseAbp which takes a startup module to initialize ABP framework. Note that this should only be done if ABP is not initialized in another way.