Assessing policy implementation and health systems impacts of Option B+ in three African countries to inform the delivery of Universal Test and Treat. In 2013, the World Health Organisation (WHO) recommended initiation of lifelong antiretroviral therapy (ART) for all HIV-positive pregnant women, regardless of disease stage, in order to minimise transmission risks to their index offspring and offspring of later pregnancies. HIV-exposed infants should receive antiretroviral (ARV) prophylaxis, be tested and receive ART if HIV-positive. In 2015, WHO extended their guidance to recommend immediate ART initiation for all adults diagnosed with HIV (universal Test and Treat (UTT)), following evidence from randomised control trials demonstrating that it reduced sexual transmission and provided individual health benefits for HIV-positive adults.  Although few sub-Saharan African countries have introduced UTT policies, many have rolled out Option B+, despite ongoing debates over its cost-effectiveness and health systems impacts, particularly in settings with weak infrastructure. Despite its potential to eliminate HIV in infants and improve maternal health, some argue that poor delivery could lead to resources being channelled away from adult HIV services, detrimentally affecting their quality and health outcomes.  There is widespread agreement that research is needed to understand how Option B+ has been implemented in different settings and its corresponding impacts on health systems. Furthermore, there is a key window of opportunity to use the evidence generated by such research to assess how best to prepare these health systems for further expansion of ART services as UTT policies are rolled out.  Our study will take place in three HIV community cohort (HCC) sites in rural Malawi, Tanzania and South Africa, representing early, mid-term and late adopters of Option B+ in 2011, 2013 and 2015 respectively, and where UTT policies were being drafted in 2016, in order to answer 4 key research questions:  1) What are the Option B+ policy implementation gaps in each setting and how do the actors, policy content, context and processes explain these gaps?  2) Have underlying economic and epidemiological assumptions in economic evaluations that demonstrated the anticipated cost-effectiveness of Option B+ been met in each site?  3) What are the health systems impacts of Option B+ in each setting?  4) What strategies can be developed with policymakers to ensure that health systems are ready for effective delivery of UTT?  We will use a comparative, longitudinal approach with mixed methods that include secondary analysis of existing policy reviews, health facility survey data and routine HIV clinic data linked to HCC data. We will also collect new data through an updated review of national HIV policies, a further round of facility surveys, key informant interviews with policymakers and programmers, in-depth interviews with health workers and PMTCT service users, and costing estimates.  We will draw on existing  frameworks to identify gaps in Option B+ implementation and to explore the policy processes and contexts underlying them. We will then assess whether the costs of implementing Option B+ in purposively sampled health facilities correspond with pre-implementation estimates, and whether epidemiological parameters used for economic evaluations prior to Option B+ implementation align with local estimates derived from the HCC. We will also adapt existing indicators to assess the impacts of Option B+ on governance, financing, service delivery, workforce, information, medical supplies, and use qualitative and HCC data to consider impacts on health systems processes commonly defined as those relating to access, quality and coverage. Using our findings, we will work with key stakeholders to develop tools to assess health systems readiness for UTT, and to monitor health systems impacts through its implementation.   The research questions and methods were developed in collaboration with policymakers to support the uptake of the findings into UTT policies in each country and beyond. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.