Textbox Scrolling in WPF

Question

This seems to be a problem I struggle with on a recurring basis. I've got a control that has a hierarchy like:

<grid>
    <stackpanel>
        <expander>
            <textbox>

The problem is that when a bunch of text is bound to the textbox, it just expands off into infinity. The behavior I want of course is for the textbox to fill up the available space in the visible window and throw up a scroll bar to allow the user to see all of the content.

Can someone clue me into the correct way to get this set up to achieve desired behavior?