The controller for path was not found or does not implement IController

Yet another possible root cause for this error is if the namespace for the area registration class does not match the namespace for the controller.

E.g. correct naming on controller class:

namespace MySystem.Areas.Customers
{
    public class CustomersController : Controller
    {
        ...
    }
}

With incorrect naming on area registration class:

namespace MySystem.Areas.Shop
{
    public class CustomersAreaRegistration : AreaRegistration
    {
        ...
    }
}

(Namespace above should be MySystem.Areas.Customers.)

Will I ever learn to stop copy and pasting code? Probably not.

I've found it.

When a page, that is located inside an area, wants to access a controller that is located outside of this area (such as a shared layout page or a certain page inside a different area), the area of this controller needs to be added. Since the common controller is not in a specific area but part of the main project, you have to leave area empty:

@Html.Action("MenuItems", "Common", new {area="" }) 

The above needs to be added to all of the actions and actionlinks since the layout page is shared throughout the various areas.

It's exactly the same problem as here: ASP.NET MVC Areas with shared layout

Edit: To be clear, this is marked as the answer because it was the answer for my problem. The above answers might solve the causes that trigger the same error.

Also, for those who the solution above didn't work, here's is what worked for me:

I have a solution with multiple projects. All projects were in MVC3. I installed Visual Studio 2012 in my machine and it seems that some projects were automatically upgraded to MVC4.

I got this problem

The controller for path '/etc/etc' was not found or does not implement IController

because the project that handled that route was pointing to MVC4.

I had to manually update their references to use MVC3. You can also do that by opening the .csproj file with a text editor. Find the reference to MVC3 and remove this line:

<SpecificVersion>False</SpecificVersion>

In my case, the same error was not related to Area but thought to post the error caused in my case, which may be helpful for the people who come to this thread by searching "The controller for path was not found or does not implement IController"

The error was caused because of wrong entry in _Layout.cshtml file.

@Styles.Render("~/Content/misc")

The bundle with that name was removed in BundleConfig.cs but forgot to remove it in _Layout.cshtml

It was silly, but we programmers always do lot of silly mistakes :)