Migrating a technology stack to a new language is rarely a simple task. It’s getting even more challenging when what is changed is not only the language but the whole paradigm. This talk covers a story of stylefruits, where we’ve been gradually replacing a Ruby-based technology stack serving five million monthly visitors with Clojure. What are the costs and benefits of such a transition? How to make the migration gradual and painless? How to make Ruby and Clojure work with each other on the way? How easy is it to switch from a dynamic, object-oriented language to a functional one based on immutability and laziness? These are just some takeaways from this straight-from-the-trenches report.