The purpose of this research is to develop a new methodology based upon ideas on managing complexity from the Viable System Model. The context for the research is Small and medium-sized enterprises (SMEs) in Mexico. Worldwide, SMEs represent the segment of the economy that contributes the largest number of economic units and employees, both in industrialised countries and in those that are less developed. However, the astonishing rate of change today influences most human activities, including business organisations, and, therefore SMEs. Organisational complexity continues to grow as organisations are forced to address more issues and greater diversity in their operating environments. So, the current challenges imposed by modern-day complexity suggest to think about new ways of approaching managementpractice. The research aims to adopt systems thinking approaches applied on daily life as an ongoing process, based on a learning system which aims to increase the ability to manage complexity in SMEs to last over time. The research design is based on an action research approach developing a single case study intervention, based on Yin's work, in a Mexican SME in order to provide the empirical data. To do so, this work presents a novel model (ModK+) and multi-methodology (MetK+) as a way of thinking and acting, respectively, to perform a systemic intervention, linking the philosophical, methodological and practical levels. Finally, and based on the sources of evidence, the researcher realised two main findings. First, the MetK+ facilitated the adoption of systems thinking approaches in the daily practice of organisational management: it helped managers to identify and to overcome their main challenges and it enabled them to better manage their complexity. Second, the researcher identified the positive impact of building a learning system because it helped managers to refine their learning cycle to manage complexity; however, despite having such a learning system it was clear that managers would still require further accompaniment after the systemic intervention to overcome inertia in their busy daily agenda
