How to add checkbox to existing administration page For a custom module i would love to add a checkbox to "BANDWIDTH OPTIMIZATION" under "admin/config/development/performance". I know how to add a new admin page but this is just one checkbox and i think it would fit more there.

Can anyone tell me how to do that?
To alter a form implemented by another module, you just implement hook_form_alter(). You probably need to change the #weight property of the form element you are adding, to avoid it is rendered in the wrong place.

function mymodule_form_system_performance_settings_alter(&amp;$form, &amp;$form_state) {
 // Add the checkbox to $form.
}