In this paper, we shall review two kinds of emergent complexity in agent-based computational economics (ACE). The first kind is based on the complex systems initiated in the 1980s or even earlier by mathematicians and physicists, whereas the second kind is based on the idea of complex adaptive systems composed of autonomous agents, for which many representative works have been collected in Rosser. For the latter, we shall go further to examine the two elements which have just recently been incorporated in agent-based economic modelling, namely, intelligence and modularity. This augmentation leads to the development of neurocognitive software agents, which can guide the generation and direction of future ACE studies with a multistage 'brain/behaviourto-emergency-to-brain/behaviour' approach.