Globally, one in three women will experience intimate partner violence in their lifetime. The impact of violence on health and wellbeing is significant, yet, recognition of the relationship between intimate partner violence and ill-health is poor. Implementing and sustaining effective health care responses to intimate partner violence has proven challenging internationally. New Zealand holds a leading role via the Ministry of Health Violence Intervention Programme. Infrastructure supporting health professionals to respond to intimate partner violence and child abuse and neglect has been implemented across hospitals and selected community settings nationally. However, engagement with the primary care sector has been limited. In this study I explore what affects a sustainable response to intimate partner violence within New Zealand primary care settings. I utilise complexity theory to reconceptualise a sustainable primary care response to intimate partner violence as a complex adaptive system. Concurrently, I analyse the function of health system documents informing intimate partner violence responsiveness and interview primary care professionals on responding to intimate partner violence as a health issue. I apply a complexity-led discourse analysis to each data source to explore how system interactions at policy and practice levels influence intimate partner violence responsiveness. I then thread the data sources together to show how sustainable responses to intimate partner violence are emergent from interactions throughout the health system. This thesis argues the under-utilisation of the primary care sector in reducing intimate partner violence is a profound system gap. My use of complexity theory as an innovative qualitative research methodology calls attention to key system interactions challenging the emergence of intimate partner violence responsiveness in primary care. Similarly, I show how complexity may be harnessed to promote intimate partner violence responsiveness. This thesis shows why we need to think differently about how we work to reduce intimate partner violence. Primary care professionals are willing to address intimate partner violence, the next step is to assist the sector in being able to.