Work engagement is associated with important individual and organisational outcomes (e.g., employee health and well-being, performance). This narrative systematic review aims to synthesise the increasing number of work engagement interventions and inform future research by exploring: (1) the specific intervention foci, delivery methods and content of engagement interventions; (2) intervention effectiveness; and (3) underlying mediators and moderators. A systematic search for interventions employing a validated engagement measure revealed 40 studies. Five were personal resource building, twelve job resource building, three leadership training, eighteen health promotion, and two job and personal resource building. Twenty (50%) studies observed significant positive effects on work engagement, two (5%) had a negative effect, and eighteen (45%) had no effect. Job and personal resources, job demands and well-being were important mediators. Moderators included the specific intervention focus and delivery method, employee participation, manager support, and intervention level (top-down vs bottom-up). Bottom-up interventions, and job crafting and mindfulness interventions particularly, were most successful. Implementation difficulties were common, including poor response and attrition rates, and adverse factors (e.g. organisational restructuring, redundancy, economic downturn). We highlight implications for research and practice and stress the need to test underlying theories to build knowledge around how, why, and when interventions work. Effectiveness of work engagement interventions new, in-depth insights and knowledge around the characteristics of engagement interventions, how and why they work, and the difficulties faced during their implementation. We provide a more detailed synthesis of the specific intervention foci, delivery methods and content of interventions, their quality (a term we use here to refer to study design & associated factors such as sample sizes & type of control group), degree of successful implementation and effectiveness. These factors are, as yet, underexplored in the literature. This knowledge is important for guiding the efficient deployment of resources towards interventions appropriate for specific contexts and participants and which are most likely to yield positive results. This review responds to a call by Bailey, et al. (2015) to further knowledge around which engagement interventions are most effective and under which conditions. The literature clearly demonstrates the benefit of a qualitative exploration of how and why interventions work, which can allow, for instance, a more detailed exploration of study quality and degree of intervention implementation. Nielsen and Miraglia (2017) highlight how qualitative evaluations prevent erroneous conclusions from purely statistical evaluations, guide analysis of underlying mediators and moderators, and can investigate what kinds of intervention components work in different contexts, and why. For example, need satisfaction could be a mediator ...