<TreeView ItemsSource="{Binding FirstGeneration}">
    <TreeView.Resources>
      <DataTemplate DataType="{x:Type local:AccountNodeVM}">
        <TreeViewItem Header ="{Binding Name}" />
      </DataTemplate>

      <HierarchicalDataTemplate DataType="{x:Type local:CategoryNodeVM}" ItemsSource="{Binding SubCategories}">
        <TreeViewItem  Header="{Binding Name}" ItemsSource="{Binding Accounts}" />
      </HierarchicalDataTemplate>
    </TreeView.Resources>

    <TreeView.ItemContainerStyle>
      <Style TargetType="{x:Type TreeViewItem}">
        <Setter Property="IsExpanded" Value="{Binding IsExpanded, Mode=TwoWay}" />
        <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}" />
        <Setter Property="FontWeight" Value="Normal" />
      </Style>
    </TreeView.ItemContainerStyle>
  </TreeView>