In response to the human resource challenges facing African health systems, there is increasing involvement of informal care providers in HIV care. Through social and institutional interactions that occur in the delivery of HIV care, linkages between formal and informal systems of care often emerge. Based on a review of studies documenting the relationships between formal and informal HIV care in sub-Saharan Africa, we suggest that linkages can be conceptualised as either 'actor-oriented' or 'systems-oriented'. Studies adopting an actor-oriented focus examine hierarchical working relationships and communication practices among health systems actors, while studies focusing on systems-oriented linkages document the presence, absence or impact of formal inter-institutional partnership agreements. For linkages to be effective, the institutional frameworks within which linkages are formalised, as well as the ground-level interactions of those engaged in care, ought to be considered. However, to date, both actor- and system-oriented linkages appear to be poorly utilised by policy makers to improve HIV care. We suggest that linkages between formal and informal systems of care be considered across health systems, including governance, human resources, health information and service delivery in order to improve access to HIV services, enable knowledge transfer and strengthen health systems.