In 2009, I first learned about Event Sourcing and Command Query Responsibility Seggregation (CQRS) at a training Greg Young gave in Utrecht, The Netherlands. I remember being awed by the scalability and architectural simplicity those styles provided. However, I also remembered the technical complexity that comes with it. In 2012, I was in charge of transitioning a CQRS-based system to Event Sourcing. I knew it would be non-trivial, but boy was I in for a surprise. So, over the last four years, I've experienced first-hand how a large group of developers had to deal with the transition. It's a brilliant solution for high-performance or complex business systems, but you need to be aware that this also introduces challenges most people don't tell you about. In this talk, I'd like to share you some of the most powerful benefits of ES, but also show you the flipside of the coin and cover some of the smaller and bigger challenges you'll run into it. Again, I love it and would apply it again without any doubt, but I really want you to understand the trade-offs before you jump on the Event Sourcing train.