There are numerous attempts to design interfaces and abstractions for event handling in reactive applications. A common solution is to provide a set of combinator functions to manipulate and compose event streams, a technique originating in functional programming. Rich combinator libraries are essentially a domain specific language for common event combinations. Another kind of interface for reactive abstractions are signal expressions, a syntactical construct to combine and transform signals – time changing values. Each library has developed its own set of interfaces for reactive programming, often influences by the interface of earlier libraries, and the actual need to support specific case studies. However, no systematisation of the developed interfaces exists. In this paper, we analyze the different interfaces provided by reactive languages. We group features into several categories which relate to usages of reactive programming libraries, and discuss how the choice of features influences the expressiveness the respective library, and which implications interfaces have on the language implementation.