outputing text to a richtextbox. (NewLine?)

[Jes|er]

I need to output to a richtextbox. The problem is that when I use:

rtbMain.text = "First Line"
rtbMain.text = rtbMain.Text & "Second Line"

Puts all the text on one line. And:

rtbMain.text = "Line One"
rtbMain.text = "Line two"

Overwrites the the first.

So I need to know how to create a newline to output text to.
Or atleast simulate the enter key being pressed so I can get to the next line.