Software systems are complicated, and the scientific and engineering methodologies for software development are relatively young. We need robust methods for handling the ever-increasing complexity of software systems that are now in every corner of our lives. In this paper we focus on asynchronous event-based reactive systems and show how we start from the requirements, move to actor-based Rebeca models, and formally verify the models for correctness. The Rebeca models include the details of the signals and messages that are passed at the network level including the timing, and can be mapped to the executable code. We show how we can use the architecture design and structured requirements to build the behavioral models, including Rebeca models, and use the state diagrams to write the properties of interest, and then use model checking to check the properties. The formally verified models can then be used to develop the executable code. The natural mappings among the models for requirements, the formal models, and the executable code improve the effectiveness and efficiency of the approach. It also helps in runtime monitoring and adaptation.