Although widely employed in both object-oriented software engineering and user interface design, use cases are not well-defined. Relatively little attention has been paid to the various styles for writing the narratives that define use cases and their consequences for user interface design and software usability. Common narrative styles are presented with examples and discussions of their relative advantages and disadvantages. Essential use cases, a variant employed within usage-centered design, are contrasted with conventional use cases and scenarios. For the most efficient support of user interface design and particularly for large, complex projects, a highly-structured form of use case has evolved. New narrative elements and relationships among use cases are introduced. These include means for expressing partial or flexible ordering of interaction, relationships with business rules, as well as a clarification of the often misunderstood concept of extension that recognizes two distinct forms: synchronous and asynchronous extensions.