What is the WPF equivalent for the FlowLayoutPanel?

Maybe Wrap panel will help.

<WrapPanel Orientation="Horizontal">
    <Button Margin="3">Button 1</Button>
    <Button Margin="3">Button 2</Button>
    <Button Margin="3">Button 3</Button>
    <Button Margin="3">Button 4</Button>
    <Button Margin="3">Button 5</Button>
</WrapPanel>