http://dotnetslackers.com/articles/silverlight/Using-the-TreeView-control-in-Silverlight-4.aspx
<UserControl x:Class="TreeViewDemo.MainPage"
xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"
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:DesignHeight="600" d:DesignWidth="400">
<UserControl.Resources>
<sdk:HierarchicalDataTemplate x:Key="ContentsTemplate">
<TextBlock FontStyle="Italic" Text="{Binding Title}"/>
</sdk:HierarchicalDataTemplate>
<sdk:HierarchicalDataTemplate x:Key="GenreTemplate" ItemsSource="{Binding Contents}" ItemTemplate="{StaticResource ContentsTemplate}">
<TextBlock Text="{Binding Genre}" Margin="5,0"/>
</sdk:HierarchicalDataTemplate>
</UserControl.Resources>
<Grid>
<sdk:TreeView Margin="10,10,21,175" Name="TreeView1" ItemTemplate="{StaticResource GenreTemplate}" Background="Beige" ItemsSource="{Binding}" SelectedItemChanged="TreeView1_SelectedItemChanged"/>
<Button Content="Button" Height="23" HorizontalAlignment="Left" Margin="136,441,0,0" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
</Grid>
</UserControl>