Use case modeling is gaining increasing interest in computer-based systems engineering, especially in the earliest stages of system development, where requirements are elicited, documented and validated. This paper presents a conceptual framework for use case modelling and a new use case model with graphical representation, including support for different abstraction levels and mechanisms for managing large use case models. Current application of use cases in requirements engineering is discussed, as well as ontological and methodological issues related to use case modelling. The presented use case model has three levels of abstraction: Environment Level, relating use cases with actors and services; Structure Level, describing the episode structure of each use case; Event Level, describing stimuli, responses and system actions of each episode. All levels have graphical representation, and the paper gives examples of each level and explains the notation. The event level is described by an extension of Message Sequence Charts [ITU-T recommendation Z.120], with operators for alternative, repetition, exception and interrupt. Conclusions are given together with a discussion on further research.