To specify if the text should wrap to the next line if it is wider than the Width property
<UserControl x:Class='SilverlightApplication3.MainPage' xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' xmlns:d='http://schemas.microsoft.com/expression/blend/2008' xmlns:mc='http://schemas.openxmlformats.org/markup-compatibility/2006' mc:Ignorable='d' d:DesignWidth='640' d:DesignHeight='480'> <TextBlock TextWrapping="Wrap" Text="I think, therefore I am." FontFamily="Comic Sans MS" FontSize="20" FontWeight="Bold" Height="150" Width="180" /> </UserControl>