user1612: I would like to connect two Arduinos by a serial connection. Is it sufficient to connect TX1<-->RX2 and RX1<-->TX2 or do I also need to connect the GND of the two Arduinos?

user304: Short answer: Yes.
Rx/Tx is referenced with a logic ground. Without ground being common, there is no way to determine what is high or low.
(Sorry I don't have a more technical explanation)
Edit: I thought I would add, if you are using the same power source, then they probably have a common ground. In this case, a common power source doesn't mean the wall side of a wall adapter, but the SAME wall adapter. 

1612: <p>I would like to connect two Arduinos by a serial connection. Is it sufficient to connect TX1&lt;-->RX2 and RX1&lt;-->TX2 or do I also need to connect the GND of the two Arduinos?</p>
 304: <p>Short answer: Yes.</p>

<p>Rx/Tx is referenced with a logic ground. Without ground being common, there is no way to determine what is high or low.</p>

<p>(Sorry I don't have a more technical explanation)</p>

<p>Edit: I thought I would add, if you are using the same power source, then they probably have a common ground. In this case, a common power source doesn't mean the wall side of a wall adapter, but the SAME wall adapter. </p>
