There is an increasing demand for deploying humanitarian workers to conflict-affected areas. However, this need has expanded the risk of violent attacks against staff in insecure field settings. In this brief, we identify six country-level factors that can influence attacks on aid workers. These six factors help us to better understand the causes of aid worker attacks, and may ultimately guide towards their prevention in the future. This policy brief is the first from an emerging PRIO research program on Humanitarian Security.