Comprehensive reviews of health system strengthening (HSS) interventions are rare, partly because of lack of clarity on definitions of the term but also the potentially huge scale of the evidence. We reflect on the process of undertaking such an evidence review recently, drawing out suggestions on definitions of HSS and approaches to assessment, as well as summarising some key conclusions from the current evidence base. The key elements of a clear definition include, in our view, consideration of scope (with effects cutting across building blocks in practice, even if not in intervention design, and also tackling more than one disease), scale (having national reach and cutting across levels of the system), sustainability (effects being sustained over time and addressing systemic blockages), and effects (impacting on health outcomes, equity, financial risk protection, and responsiveness). We also argue that agreeing a framework for design and evaluation of HSS is urgent. Most HSS interventions have theories of change relating to specific system blocks, but more work is needed on capturing their spillover effects and their contribution to meeting overarching health system process goals. We make some initial suggestions about such goals, to reflect the features that characterise a "strong health system." We highlight that current findings on "what works" are just indicative, given the limitations and biases in what has been studied and how, and argue that there is need to rethink evaluation methods for HSS beyond finite interventions and narrow outcomes. Clearer concepts, frameworks, and methods can support more coherent HSS investment.