Software-intensive safety critical systems are becoming more and more widespread and are involved in many aspects of our daily lives. Since a failure of these systems could lead to unacceptable consequences, it is imperative to guarantee high safety standards. In practice, as a way to handle their increasing complexity, these systems are often modelled as hierarchical systems. To date, a good deal of work has focused on the definition and analysis of hierarchical modelling languages and on their integration within model-driven development frameworks. Less work, however, has been directed towards formalisms to effectively express, in a precise and rigorous way, relevant behavioural properties of such systems (e.g.: safety requirements). In this work, we propose a novel extension of classic Linear Temporal Logic (LTL) called Hierarchical Linear Temporal Logic (HLTL), designed to express, in a natural yet rigorous way, behavioural properties of hierarhical systems. The formalism we propose does not commit to any specific modelling language, and can be used to predicate over a large variety of hierarchical systems.
