Based on interviews with 25 engineers, in which we discussed 19 event sourced systems, we explore the rationale for and the context of event sourcing. Why do people use event sourcing, in what kind of systems do they apply it, and how satisfied are they. We also identify five challenges that practitioners experience: event system evolution, the steep learning curve, lack of available technology, rebuilding projections, and data privacy. We dig deeper into the first challenge, event system evolution. We uncover five tactics that engineers use in upgrading their event sourced system: versioned events, weak schema, upcasting, in-place transformation, and copy-and-transform. In this talk we share the results of our scientific study on event sourcing. We do not share the experience of one company, or one product, but the accumulated experiences of 25 engineers.