Question: I don't know why I cleaned the project, but I did, and now one of my activities is reporting 50 errors, like:

```
    setContentView(R.layout.activity_settings);
    // activity_settings cannot be resolved or is not a field

    CheckBox check_sound = (CheckBox)findViewById(R.id.settings_sound);
    // settings_sound cannot be resolved or is not a field

```

And so on. This was working fine before I cleaned the project and nothing has been deleted. Only one activity has errors, no other errors reported in any other file. Tried restarting, Build automatically is enabled, Project>Build and Project>Build All are greyed out.
What's going on?

Answer: 
Found the problem. For some reason, the first import in the problematic activity was `import android.R`. I don't know how/why it got there. I ran Lint and went through every one of the entries/errors/warnings until I deleted `import android.R` out of desperation. Suddenly everything started working properly again.

Of course when I Googled this solution, I found several answers but for some reason I could not find them before I found the answer myself.

I swear no single programming problem in any of my apps was even close to problems I had dealing with Eclipse/ADT. From a simple SDK update, to reinstallation, to nonsense like this,
@timsim

1) To do so just right click on yout Android project:

* Android tools
* Clear Lint Markers
It worked perfectly for me.

2) try to check preferences->Android->SDK location. In some case, Android sdk requires higher version of ADT. Refer to this link: This Android SDK requires Android Developer Toolkit version 14.0.0 or above. .... Please update ADT to the latest version

Let me know if you face any issue.

Thanks