This paper introduces Live Sequence Charts (LSCs), an extension of Message Sequence Charts (MSCs) designed to enhance their expressive power and rigor. LSCs are proposed to address the limitations of MSCs, which are widely used in industry but lack a clear semantic foundation and expressive power for specifying mandatory behavior. The authors argue that LSCs can distinguish between possible and necessary behavior, both globally and locally, and support the specification of forbidden scenarios. They also aim to bridge the gap between inter-object and intra-object specification in system behavior, a central problem in behavioral specification. LSCs introduce the concept of "liveness," allowing users to specify mandatory behavior and conditions that must be met for the system to adhere to the specified scenario. This is achieved by labeling elements as "hot" (mandatory) or "cold" (provisional), and by associating activation conditions with charts. The paper outlines the syntax and semantics of LSCs, including the ability to specify forbidden scenarios and the use of subcharts, branching, and iteration. The authors demonstrate the practical application of LSCs through an example from the rail-car system, showing how LSCs can capture both possible and necessary behaviors, and how they can be used to specify scenarios that must always be adhered to once certain conditions are met. The paper concludes by highlighting the potential of LSCs in advancing the automation and reliability of system development.This paper introduces Live Sequence Charts (LSCs), an extension of Message Sequence Charts (MSCs) designed to enhance their expressive power and rigor. LSCs are proposed to address the limitations of MSCs, which are widely used in industry but lack a clear semantic foundation and expressive power for specifying mandatory behavior. The authors argue that LSCs can distinguish between possible and necessary behavior, both globally and locally, and support the specification of forbidden scenarios. They also aim to bridge the gap between inter-object and intra-object specification in system behavior, a central problem in behavioral specification. LSCs introduce the concept of "liveness," allowing users to specify mandatory behavior and conditions that must be met for the system to adhere to the specified scenario. This is achieved by labeling elements as "hot" (mandatory) or "cold" (provisional), and by associating activation conditions with charts. The paper outlines the syntax and semantics of LSCs, including the ability to specify forbidden scenarios and the use of subcharts, branching, and iteration. The authors demonstrate the practical application of LSCs through an example from the rail-car system, showing how LSCs can capture both possible and necessary behaviors, and how they can be used to specify scenarios that must always be adhered to once certain conditions are met. The paper concludes by highlighting the potential of LSCs in advancing the automation and reliability of system development.