This paper introduces Live Sequence Charts (LSCs), an extension of Message Sequence Charts (MSCs) that allows for the specification of both possible and necessary behavior in system scenarios. LSCs address the limitations of MSCs, which are based on partial ordering of events and lack the ability to distinguish between possible and necessary behavior. LSCs introduce the concept of "liveness," enabling the specification of mandatory behavior, such as ensuring that certain events must occur. This allows for the distinction between possible and necessary behavior at both the global level of an entire chart and locally within specific events, conditions, and progress over time. LSCs also support the specification of forbidden scenarios, enhancing structuring constructs like subcharts, branching, and iteration. The paper discusses the need for a highly expressive and rigorously defined MSC language to support serious, semantically meaningful tool support for use-cases and scenarios. It also highlights the importance of relating scenario-based inter-object specification to state-machine intra-object specification, a central problem in behavioral system specification. The paper proposes LSCs as a solution, which extend the ITU standard for MSCs by allowing parts of a chart to be designated as live or mandatory. This enables the specification of mandatory behavior, such as ensuring that certain messages are sent and received, progress is made, etc. LSCs also allow for the specification of conditions as first-class citizens, enabling the definition of both mandatory and provisional conditions. Mandatory conditions require that certain behaviors must occur, while provisional conditions allow for the possibility of certain behaviors. The paper also discusses the semantics of LSCs, defining how they relate to system behavior and how they can be used to specify and analyze use-cases and scenarios. It presents an example of LSCs applied to a rail-car system, illustrating how they can be used to specify different scenarios and ensure that the system adheres to the required behavior. The paper concludes that LSCs provide a powerful and expressive way to specify system behavior, enabling the construction of semantically meaningful computerized tools for describing and analyzing use-cases and scenarios.This paper introduces Live Sequence Charts (LSCs), an extension of Message Sequence Charts (MSCs) that allows for the specification of both possible and necessary behavior in system scenarios. LSCs address the limitations of MSCs, which are based on partial ordering of events and lack the ability to distinguish between possible and necessary behavior. LSCs introduce the concept of "liveness," enabling the specification of mandatory behavior, such as ensuring that certain events must occur. This allows for the distinction between possible and necessary behavior at both the global level of an entire chart and locally within specific events, conditions, and progress over time. LSCs also support the specification of forbidden scenarios, enhancing structuring constructs like subcharts, branching, and iteration. The paper discusses the need for a highly expressive and rigorously defined MSC language to support serious, semantically meaningful tool support for use-cases and scenarios. It also highlights the importance of relating scenario-based inter-object specification to state-machine intra-object specification, a central problem in behavioral system specification. The paper proposes LSCs as a solution, which extend the ITU standard for MSCs by allowing parts of a chart to be designated as live or mandatory. This enables the specification of mandatory behavior, such as ensuring that certain messages are sent and received, progress is made, etc. LSCs also allow for the specification of conditions as first-class citizens, enabling the definition of both mandatory and provisional conditions. Mandatory conditions require that certain behaviors must occur, while provisional conditions allow for the possibility of certain behaviors. The paper also discusses the semantics of LSCs, defining how they relate to system behavior and how they can be used to specify and analyze use-cases and scenarios. It presents an example of LSCs applied to a rail-car system, illustrating how they can be used to specify different scenarios and ensure that the system adheres to the required behavior. The paper concludes that LSCs provide a powerful and expressive way to specify system behavior, enabling the construction of semantically meaningful computerized tools for describing and analyzing use-cases and scenarios.