Every massive structure has basic units that come together and make sense to the whole structure. For example, bricks, cement, paint, concrete, and so on make up basic building blocks on a building. Similarly, before we go ahead and discuss the massive field of functional reactive programming, it would be great if we understood the basic building blocks that will come together so that the massive applications that we create make sense. In the next subsections, we will cover the basic building blocks of FRP, starting with event streams.