Assume that a group of organizations decide to integrate their online businesses. One (or more) of them will provide a front end that enables user interaction via Web. Modeling interactions among software components is becoming increasingly important for analyzing behavior of web software. This is especially true for the area of web services where distributed interactions across organizational boundaries are the norm. In this paper, the author will discuss three visual formalisms for modeling interactions: collaboration diagrams, message sequence charts, and conversation protocols. The author will discuss the differences and similarities among these representations and identify two problems: realizability and synchronizability.