KeywordsAdolescent, child, child abuse, child participation, child protection, childhood violence, drivers of violence, South East Asia, epidemiology, Indonesia, risk and protective factors, sexual abuse, sexual violence.ii Challenges to researching childhood sexual violence in Indonesia Abstract Background: Childhood sexual violence (CSV) is a global child rights and public health concern, with severe consequences for children and society. However, little is known about CSV in low-and middle-income countries. Indonesia has the largest Muslim population in the world and is home to almost 90 million children. To date, there is limited data from this vast country about the nature of CSV, including the prevalence, associated risks, and protective factors. Available data is of mixed quality and inadequate to inform policy. This research has three aims: i) to collect and review available data about CSV in Indonesia; ii) to analyse the lessons learned from existing efforts to research CSV in Indonesia, with specific reference to Indonesia's recent population survey on childhood violence Challenges to researching childhood sexual violence in Indonesia iii Results: The first phase of the research, the systematic literature review, found that the available data provides an inadequate picture of CSV in Indonesia. Prior studies have generally been small in scale and unrepresentative. Previous efforts to collect survey data on childhood violence lack validity or sufficient information to inform response efforts. Prevalence estimates vary widely, depending on the definition and methodology used. Despite the heterogeneity of the studies identified in the review, several patterns emerged. Available research points to a significant, but underreported problem particularly affecting adolescent girls in relationships.Exposure to childhood violence (including harsh parenting and witnessing domestic violence) is strongly associated with re-victimisation and perpetration in adulthood. Few child victims disclose incidents or seek support. There is limited data about risk and protective factors available across the life course, while no data is available about sexual violence in early childhood. The varied quality of the studies included in the review highlight the need for urgent investment in national capacity to conduct qualitative and quantitative research on childhood violence, including sexual violence, in Indonesia to better inform policy and prevention programs, and to justify increased investment into services for child victims (Rumble, Febrianto, et al., 2018).The second phase of the research built upon the systematic review and provides practical insights from the Violence against Children Survey (Kurniasari et al, 2014). It illustrates the importance of contextual factors (such as language, religion, and cultural barriers to disclosure) when designing and carrying out future population based research (Rumble, Ramly, Nuryana,