The Multi-Agent-Based Simulation (MABS) workshop is the twentieth of a series that began in 1998 (http://www.pcs.usp.br/~mabs/). Its scientific focus lies in the confluence of social sciences and multi-agent systems, with a strong application/empirical vein, and its emphasis is stressed on (i) exploratory agent based simulation as a principled way of undertaking scientific research in the social sciences and (ii) using social theories as an inspiration to new frameworks and developments in multi-agent systems.