To reach the third 90 of the UNAIDS 90–90–90 targets, country programmes must delve into their data and understand how they represent the quality of viral load (VL) testing services. This guide, published by WHO, presents key considerations and examples of tools (provided in the annexes) to assist countries in developing a national VL M&E plan. As countries invest in scaling up of routine VL testing, it is critical to measure the impact of and progress towards achieving the UNAIDS target that 90% of people receiving antiretroviral therapy (ART) have suppressed viral loads by 2020 (as part of the 90–90–90 targets). It has three sections that cover: 1) assessing monitoring and evaluation (M&E) data systems and tools, and understanding how data flows to and from facilities, 2) indicators that M&E systems include longitudinal follow up of unsuppressed VL, and 3) methods for evaluating VL implementation plans and examples of evaluation questions.