In this chapter we have considered tasks that touch several layer of the MVC architecture. First we have seen how to cover extensions for multi languages by adjust static text in the templates and also country-specific formats like date or time are adjusted. In addition we have shown that also the domain model itself can be translated. After this we show the check of invariants in the model, inclusive the display of error messages in the template and the possibility to correct wrong entries by the user. At last we have shown concepts that apply to the security of the extension. Important by this is tha you have understand what are request hashes and how to prevent against cross site scripting.