Despite growing support for integration of frontline services, a lack of information about the pre-conditions necessary to integrate such services hampers the ability of policy makers and implementers to assess how feasible or worthwhile integration may be, especially in low- and middle-income countries (LMICs). We adopted a modified systematic review with aspects of realist review, including quantitative and qualitative studies that incorporated assessment of health system preparedness for and capacity to implement integrated services. We searched Medline via Ovid, Web of Science and the Cochrane library using terms adapted from Dudley and Garner’s systematic review on integration in LMICs. From an initial list of 10 550 articles, 206 were selected for full-text review by two reviewers who independently reviewed articles and inductively extracted and synthesized themes related to health system preparedness. We identified five ‘context’ related categories and four health system ‘capability’ themes. The contextual enabling and constraining factors for frontline service integration were: (1) the organizational framework of frontline services, (2) health care worker preparedness, (3) community and client preparedness, (4) upstream logistics and (5) policy and governance issues. The intersecting health system capabilities identified were the need for: (1) sufficiently functional frontline health services, (2) sufficiently trained and motivated health care workers, (3) availability of technical tools and equipment suitable to facilitate integrated frontline services and (4) appropriately devolved authority and decision-making processes to enable frontline managers and staff to adapt integration to local circumstances. Moving beyond claims that integration is defined differently by different programs and thus unsuitable for comparison, this review demonstrates that synthesis is possible. It presents a common set of contextual factors and health system capabilities necessary for successful service integration which may be considered indicators of preparedness and could form the basis for an ‘integration preparedness tool’.