Health system strengthening (HSS) has often been undertaken by global health actors working through vertical programmes. However, experience has shown the challenges of this approach, and the need to recognize health systems as open complex adaptive systems-which in turn has implications for the design and implementation approach of more 'horizontal' HSS interventions. From 2009 to 2016, the Doris Duke Charitable Foundation supported the African Health Initiative, establishing Population Health Implementation and Training partnerships in five African countries (Ghana, Mozambique, Rwanda, Tanzania and Zambia). Each partnership was designed as a large-scale, long-term, complex health system strengthening intervention, at a primary care or district level-and in each country the intervention was adapted to suit that specific health systems context. In Mozambique, the Population Health Implementation and Training partnership sought to strengthen integrated health systems management at district and provincial levels (through a variety of capacity-development intervention activities, including in-service training and mentoring); to improve the quality of routine data and develop appropriate tools to facilitate decision-making for provincial and district managers; and to build capacity to design and conduct innovative operations research in order to guide integration and system-strengthening efforts. The success of this intervention, as assessed by outcome measures, has been reported elsewhere. In this paper, the implementation practice of this horizontal HSS intervention is assessed, focusing on the key features of how implementation occurred and the implementation approach. A case study focusing on HSS implementation practice was conducted by external researchers from 2014 to 2017. The importance of an accompanying implementation research approach is emphasized-especially for HSS interventions where the 'complex adaptive system' (complex and constantly changing context) forces constant adaptations to the intervention design and approach.