This article describes a framework for requirements engineering with the so-called use case scenario models. Use case scenario models are based on Message Sequence Charts (MSC) and High-level Message Sequence Charts (HMCS). In contrast to more conventional usage, (H)MSC scenarios are used to define behavior of a single object – the system under development for which requirements are modeled. Our formalization of use case scenario models facilitates requirements validation. Requirements validation is done by first automatically synthesizing executable model from scenarios and then performing simulation and model checking of the synthesized requirements model. The objective of our approach to requirements engineering is to have: 1) intuitive yet powerful formalism for modeling requirements; 2) intuitive structure of the synthesized executable models; 3) possibility to use powerful model checking and simulation tools. In order to achieve these objectives, instances of an (H)MSC model is independently mapped to the so-called Event Automata. The collection of Event Automata for all instances in the (H)MSC model provides some approximation of (H)MSC semantics. The goal of this paper is to formalize the nature of this approximation. We show that this approximation is adequate for the purpose of requirements modeling, and emphasize the benefits of the suggested approach: intuitive formal models of requirements, simple and flexible platform for generating code and using various powerful simulator and model checking tools, easy mapping of the analysis results back to the model. We outline the formalization of the use case models, event automata and synthesized requirements models. Additionally we describe a novel Video Cassette Recorder metaphor for visualization of use case scenario models, suitable for the problem domain experts, who are not familiar with formal notations. We also describe the Use Case Studio toolkit, in which these ideas are implemented.