Key findings || The total global number of IDPs has been roughly twice that of refugees in recent years, and the gap between estimates for the two groups has been growing over the last 20 years. Data on conflict-related displacement shows that many of the top refugee-producing countries are also home to the highest numbers of IDPs. Six of the ten countries that produced the most refugees in 2016 – Afghanistan, Colombia, DRC, South Sudan, Sudan and Syria – were also among the ten with the largest numbers of IDPs. || While the return of refugees to their country of origin is often regarded as a viable and politically preferred solution, returnees may in fact return to situations of questionable security and stability and risk becoming displaced again, this time internally. The return of refugees from Pakistan to Afghanistan and the announced returns from the Dadaab refugee camp in Kenya to Somalia are cases in point. Similarly, escalating and violent conflicts such as that in South Sudan can mean that people become caught up in a revolving door of circular cross-border displacement that is difficult to monitor. || The phenomenon of IDPs moving onwards across borders is not systematically measured, and there is insufficient quantitative and qualitative data and analysis to inform effective policy and operational responses. Cross-border displacement by disasters is also not systematically recorded and while estimated to be lower in numbers, does occur and needs to be better understood. 50 GRID 2017 || Available evidence suggests that the push and pull factors for internal displacement from areas affected by conflict are similar to those reported by refugees. Overall, there is currently not enough research or data to understand the exact relationship between internal displacement, cross-border movement and return. A research and policy agenda is needed to: a) capture more accurately the scale and proportion of IDPs who cross borders, and how these vary across different contexts and crises: this requires the alignment and interoperability of data collection systems, with joint collection exercises to monitor displacement trajectories, including across borders, over longer time periods; b) understand the combination of factors that determine IDPs’ onward and cross-border flight: understanding how and when people make such decisions and the different influencing factors is a prerequisite for planning and preparedness; c) better understand the circumstances in which people return to their countries of origin, and a measure of the risk this carries for future displacement: monitoring returnees’ trajectories and gathering data on the indicators for durable solutions over time are essential.