While humanitarians and military actors have long shared operational environments, increasing support for military involvement in assistance and protection strategies is challenging the capacities and security approaches of the humanitarian system in new ways. These changes pose important questions for both humanitarian and military communities, including how to maintain the integrity of humanitarian principles in combined political, military and humanitarian efforts; how to design and resource military roles in protecting civilians from violent harm; how to manage the impact of, and hold to account, the private security industry in crisis response, and how to manage the security of aid operations generally. This paper summarises HPG’s latest Monitoring Trends report, the fourth in HPG’s annual series, which reviews these trends, and makes recommendations to the humanitarian and defence sectors to progress issues of mutual concern to both communities.