This document discusses requirements modeling in software engineering. It covers creating various models during requirements analysis, including scenario-based models, data models, class-oriented models, flow-oriented models, and behavioral models. These models form the requirements model, which is the first technical representation of a system. The document provides examples of writing use cases and constructing a preliminary use case diagram for a home security system called SafeHome. It emphasizes that requirements modeling lays the foundation for software specification and design.