Tourism is one of the prime manifestations of the 'great acceleration of humankind' since the Anthropocene started around 1950. The almost 50-fold increase in international tourism arrivals has substantial implications for environmental sustainability, but these have not yet been fully explored. This paper argues that a full exploration requires the study of tourism as a complex socio-ecological system. Such approach integrates environmental processes and stakeholder behaviour and puts feedbacks in the spotlight. Systemic insights can inform strategies to address tourism's problematic environmental performance. The paper finds that systems approaches in tourism research are rare and identifies a number of challenges: the large number of stakeholders involved; the heterogeneity of stakeholders; and the lack of transdisciplinarity in tourism research. The paper then argues that agent-based modelling can help address some of these challenges. Agent-based modelling allows to run simplified tourism systems with heterogeneous stakeholders and explore their behaviour, thus acting as living hypotheses. They do this by: ( 1) representing tourism's dynamics in a systemic, intuitive and individual-based way; ( 2) combining theories from different domains; ( 3) unpacking the link between stakeholder behaviours and emergent tourism system patterns; and ( 4) connecting researchers and stakeholders. Agent-based models allow representation of heterogeneous agents driven by plausible needs, who perceive local context and interact socially. Companion modelling is identified as a promising tool for more effective stakeholder inclusion.