Virtually all current major social and environmental challenges such as financial crises, migration, erosion of democratic institutions, or loss of biodiversity, involve complex systems comprised of decision-making, interacting, adaptive agents. To understand how such agent-based complex systems function and respond to change and disturbances, agent-based modelling (ABM) is increasingly recognized as the main way forward. Many good examples of agent-based models exist that were realistic enough to successfully support the management of complex systems, but these solutions are case-specific, there is little theory. General theory, though, is highly needed because we cannot model each and any system and question. Still, across disciplines, a critical mass of expertise has accumulated which could transform ABM into a more coherent, efficient approach to discover the functioning of complex social-economic-ecological systems. To this end, this initiative offers a platform for a cross-disciplinary discussion among researchers from various disciplines, and a goal-oriented synthesis to transform ABM into a more coherent, efficient approach to identify general principles and theories being essential to improve our understanding and management of complex systems.