Systems thinking in practice is a heuristic framework based upon ideas of boundary critique for guiding the use and development of tools from different traditions in managing complex realities. Three interrelated features of the framework are drawn out – contexts of systemic change, practitioners as change agents, and tools as systems constructs that can themselves change through adaptation. A range of tools associated with the Systems tradition have demonstrable capacity to change and adapt by continual iteration with changing context of use and different practitioners using them. It is in the practice of using such tools whilst being aware of significant ‘traps’ in managing complex realities that enables systems thinking in practice to evolve. Systems thinking can inadvertently invite traps of reductionism within contexts, dogmatism amongst practitioners, and fetishism of our tools as conceptual constructs associated with ultimately undeliverable promises towards achieving holism and pluralism. The heuristic provides a guiding framework on monitoring the development of tools from different traditions for improving complex realities and avoiding such traps.