In 2018, the Government of Yemen and the Houthis concluded the UN-mediated Stockholm Agreement in which they agreed on a ceasefire in Hodeidah to be overseen by a UN monitoring mission. As of 2020, the implementation of the ceasefire is stalled, and the humanitarian situation has not improved. The purpose of this article is to provide a descriptive analysis of the challenges that UNMHA monitors have faced in Yemen. The empirical analysis builds on the literature on ceasefires and monitoring missions and focuses on four key factors: agreement quality, changes in the operational environment, the monitoring mission’s relation to the mediator, and conflict parties’ commitment to the ceasefire. I apply a qualitative case-study method, reviewing primary and secondary sources and conducting interviews with monitoring officers and local Yemenis. I find that monitors’ ability to carry out their mandate was hampered by the quality of the agreement and conflict parties’ perception of bias. Second, I find that the Houthis, operating from a position of relative strength prevented monitors from carrying out their mandate. Findings from the Yemeni case are relevant for other monitoring missions that are deployed in ongoing violent contexts, such as Libya or Ukraine.