This paper describes the experiments that have been conducted to determine the optimal implementation method for AES ( Advanced Encryption Standard) data encryption in a ZigBee network in terms of energy consumption. Four possible scenarios have been considered. The first one is a freely available AES-cryptographic algorithm on the processor which controls the ZigBee nodes. The second also involves the processor but is based on a proprietary, highly optimized algorithm. The other methods are based on hardware implementations. Whereas the third option is based on a cryptographic block, embedded in the ZigBee node controller, the last solution utilizes an AES128-CBC-MAC hardware co-processor embedded on a Spartan 3A FPGA. The ZigBee modules in the network contain an 8-bit microcontroller which takes care of the ZigBee protocol stack-and the encryption calculations in all but the last case. All approaches are examined and compared. We show that the usage of a microcontroller with an on-board AES processor is the optimal design choice for a new hardware platform. An optimized software library gives the best results when extending an existing platform. This paper gives developers an idea of the amount of energy they can gain/lose by choosing one of the four solutions. Copyright (c) 2010 John Wiley & Sons, Ltd.