How do I get a TabControl to use the full width of its parent?

1. Remove the height and width attributes from TabControl
2. Set horizontal and vertical alignment to stretch

e.g. won't stretch;

<TabControl Height="373" Width="609" HorizontalAlignment="Stretch" Name="tabControl1" VerticalAlignment="Stretch"  VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch">

e.g. will stretch;

<TabControl HorizontalAlignment="Stretch" Name="tabControl1" VerticalAlignment="Stretch"  VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch">

Instead of using the Dock property you should try using the Anchor to anchor each of the four sides. Then you need to position the TabControl so it is positioned a couple of pixels bigger on all sides that the parent. That way the borders are hidden because they cannot be drawn when behind the parent control.