This paper is just a concept presentation to be discussed at the ECMS12, based on preliminary work of a research project funded by the Spanish Institute for Fiscal Studies (Ministry of Economy). This project aims to build an agent-based model (ABM) for the simulation of tax compliance and tax evasion behaviour, and to calibrate it empirically in order to generate some known patterns of tax behaviour among Spanish taxpayers. Here we present the state of the development for the formal model and our present ideas about the implementation methodology, with focus on a new algorithm -based in four different decisional mechanismsso that it includes not just the usual expected utility optimization, but also other sociologically relevant features like social network structure, social influence, decisional heuristics, biases in the perception of the tax system, and heterogeneity of tax motivations and tax morale among the agents. The methodological discussion about this kind of “modularity” in implementing a decisional engine could be completed in Koblenz with some preliminary results based on experimentation with the initial parameters and decisional modules.