On the basis of extensive practical experience in the development of agent-based systems for manufacturing execution and agent-based systems in general, a reference model for holonic execution known as HERA (Holonic Execution Reference Architecture) is developed. The model is characterised by a focus on holarchy - Koestler's Janus Effect (Koestler, 1967) is explicitly captured. However, the Holonic Manufacturing Systems (HMS) Consortium's view of a holon having an information processing part and a physical part (Brennan and Norrie, 2003) is also present. We refer to these parts as the behaviour and the embodiment respectively.