In this paper we present findings of a systematic review that explores a broad range of evidence on: (i) the extent and nature of the integration of targeted health programmes that emphasize specific interventions into critical health systems functions, (ii) how the integration or non-integration of health programmes into critical health systems functions in different contexts has influenced programme success, (iii) how contextual factors have affected the extent to which these programmes were integrated into critical health systems functions. Our analysis shows few instances where there is full integration of a health intervention or where an intervention is completely non-integrated. Instead, there exists a highly heterogeneous picture both for the nature and also for the extent of integration. Health systems combine both non-integrated and integrated interventions, but the balance of these interventions varies considerably.