Background:Despite evidence demonstrating the utility of using Person-Centred Outcome Measures within palliative care settings, implementing them into routine practice is challenging. Most research has described barriers to, without explaining the causal mechanisms underpinning, implementation. Implementation theories explain how, why, and in which contexts specific relationships between barriers/enablers might improve implementation effectiveness but have rarely been used in palliative care outcomes research.Aim:To use Normalisation Process Theory to understand and explain the causal mechanisms that underpin successful implementation of Person-Centred Outcome Measures within palliative care.Design:Exploratory qualitative study. Data collected through semi-structured interviews and analysed using a Framework approach.Setting/participants:63 healthcare professionals, across 11 specialist palliative care services, were purposefully sampled by role, experience, seniority, and settings (inpatient, outpatient/day therapy, home-based/community).Results:Seven main themes were developed, representing the causal mechanisms and relationships underpinning successful implementation of outcome measures into routine practice. Themes were: Subjectivity of measures; Frequency and version of Integrated Palliative care Outcome Scale; Training, education, and peer support; Building and sustaining community engagement; Electronic system readiness; The art of communication; Reinforcing use through demonstrating value.Conclusions:Relationships influencing implementation resided at individual and organisational levels. Addressing these factors is key to driving the implementation of outcome measures into routine practice so that those using palliative care services can benefit from the systematic identification, management, and measurement of their symptoms and concerns. We provide key questions that are essential for those implementing and using outcome measures to consider in order to facilitate the integration of outcome measures into routine palliative care practice.