Online sports betting is a $50B industry that is heavily driven by software. The domain imposes significant demands on developers: the resulting solutions are large, complex, distributed, concurrent software systems with strict availability, real-time performance, scalability, reliability, and security requirements. This paper describes our experience with EmpireBet, a family of online sports betting platforms built and deployed over the past 15 years. The initial solution, implemented by four developers in a start-up, catered to users who connected to the system intermittently, for limited periods, via dial-up connections. Today’s system, engineered and maintained in 27 programming and markup languages by a team of 20 developers, is deployed in over 30 countries, integrated with over 50 third-party systems, and processes tens of millions daily transactions by over 680,000 players who are continuously using the system. This was accomplished via an an explicit focus on EmpireBet’s critical non-functional requirements; a modular, extensible architecture; a set of novel abstractions we introduced into the system; and several reusable libraries developed in the process.