Android: how to create a transparent dialog-themed activity

The easiest way that I have found is to set the activity's theme in the AndroidManifest to android:theme="@android:style/Theme.Holo.Dialog" then in the activity's onCreate method call getWindow().setBackgroundDrawable(new ColorDrawable(0));

Just one thing to add to all the answers here. To achieve a full dialog like behaviour, you can reposition the dialog by changing the window layout params:

WindowManager.LayoutParams params = getWindow().getAttributes(); 
params.x = ...;
params.y = ...;
params.width = ...;
params.height = ...;

this.getWindow().setAttributes(params);

We used a similar technique to achieve a floating activity effect in the Tooleap SDK:

dialog.getWindow().setBackgroundDrawable(new ColorDrawable(0));

and import android.graphics.drawable.ColorDrawable; on top :)

You can create a tranparent dialog by this.

public class DialogActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final AlertDialog alertDialog = new AlertDialog.Builder(this).create();
        alertDialog.setTitle(" ");
        alertDialog.setMessage("");
        alertDialog.setIcon(R.drawable.icon);
        alertDialog.setButton("Accept", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {                
            }
        });
        alertDialog.setButton2("Deny", new DialogInterface.OnClickListener() {
            public void onClick(DialogInterface dialog, int which) {
            }
        });
        alertDialog.show();
    }
}

After this just put a line in AndroidManifest.xml, where you define your activity in manifest.

android:theme="@android:style/Theme.Translucent.NoTitleBar"