Paper on our experiences with writing the Xen control stack in OCaml at ICFP 2010. Working with Dave Scott, Richard Sharp, and Thomas Gazagnaire, we presented the perspectives and perceptions from using functional programming within an industrial product group at Citrix. The paper discusses the practical realities of deploying OCaml in a commercial virtualization product, covering both the technical benefits of the functional approach and the organizational challenges of introducing FP into an established engineering team. This became an influential case study for FP adoption in industry.