We derive the ǫ-outage capacity of a wireless relay network where relaying only takes place if the destination has not been able to decode the source message. We refer to this scheme as incremental relaying. The relays use space-time block coding and perform decode-and-forward. We compare incremental relaying to transmit diversity in terms of ǫ-outage capacity and signal-to-noise ratio gain. It is demonstrated that incremental relaying outperforms transmit diversity only in certain regions that determine the relay locations.