In this chapter, we learned about the reactivity of RShiny. We learned in detail how reactive source, endpoint, and conductor work, and how to control specific input with isolate(). We have also scheduled the reactive of functions. After that, we used observeEvent and eventReactive for event-handling, and learned how to make our code more modular. After developing our application, we carried out testing using Shinytest, debugging, and error-handling. On top of all this, we discussed how to improve the performance of our Shiny code using profiling.