Purpose and setting: The last decade has seen increased use of health impact assessment (HIA) to influence public policies developed outside the Health sector. HIA has developed as a structured, linear and technical process to incorporate health, broadly defined, into policy. This is potentially incongruent with complex, non-linear and tactical policy making which does not necessarily consider health. HIA research has however not incorporated existing public policy theory to explain practitioners’ experiences with HIA and policy. This research, therefore, used public policy theory to explain HIA practitioners’ experiences and investigate ‘What is the fit between HIA and public policy?’ Methods: Empirical findings from nine in-depth interviews with international HIA practitioners were reanalysed against public policy theory. We reviewed the HIA literature for inclusion of public policy theories then compared these for compatibility with our critical realist methodology and the empirical data. The theory ‘Policy Cycles and Subsystems’ (Howlett et al., 2009) was used to re-analyse the empirical data. Findings: HIAs for policy are necessarily both tactical and technical. Within policy subsystems using HIA to influence public policy requires tactically positioning health as a relevant public policy issue and, to facilitate this, institutional support for collaboration between Public Health and other sectors. HIA fits best within the often non-linear public policy cycle as a policy formulation instrument. HIA provides, tactically and technically, a space for practical reasoning to navigate facts, values and processes underlying the substantive and procedural dimensions of policy. Conclusions: Re-analysing empirical experiential data using existing public policy theory provided valuable explanations for future research, policy and practice concerning why and how HIA fits tactically and technically with the world of public policy development. The use of theory and empiricism opens up important possibilities for future research in the search for better explanations of complex practical problems.