Build up formatted text programmatically to display in a RichTextBox?

Question

What I'd like to do is build up a string with specific formatting to be displayed in a RichTextBox control.

Something like this (this doesn't completely work):

Dim testText as String = ""
testText = "Test text" & vbTab & "with a tab." & vbCrLf
testText = testText & "<b>This just prints out verbatim.  How do I make it print bold?</b>"

myRichTextBox.Text = testText

Could someone point me in the right direction?

Thanks as always.