There has been a considerable growth in interest in Health Systems Research over the last 25 years. It is only recently, however, that health systems researchers have started to examine the key conceptual tool in their armoury the notion of 'system'and only then in a sporadic fashion. This paper argues that the failure to undertake this challenge has held back the field both theoretically and in terms of practice. Critical systems thinkers have, by contrast, spent the last 30 years conducting a second-order analysis of different systems and complexity approaches, the way they employ the concept of 'system', and the difference this makes to the theories espoused and the modes of intervention endorsed. They have also explored how different systems approaches can be used in combination to bring about improvement by addressing the multi-dimensional complexity thrown up by wicked problems. What critical systems thinkers have failed to do is get their hands dirty and demonstrate, through practical application, that their approach is useful in the health systems domain. Given the significance of health systems for public well-being, and their inherent complexity, this is something that needs rectifying. If Critical Systems Thinking can support Health Systems Research with appropriate theories and methodologies, and health systems researchers are willing to involve critical systems thinkers in tackling the major issues they face, then a fruitful partnership can be forged.