You probably need to add the namespace:
<Window x:Class="UserControlTest.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:UserControlTest"
Title="User Control Test" Height="300" Width="300">
<local:UserControl1 />
</Window>