Understanding the relevant behaviour of fire in buildings is critical for the continued provision of fire safety solutions as infrastructure continually evolves. Traditionally, new and improved understanding has helped define more accurate classifications and correspondingly, better prescriptive solutions. Among all the different concepts emerging from research into fire behaviour, the compartment fire is probably the one that has most influenced the evolution of the built environment. Initially, compartmentalization was exploited as a means of reducing the rate of fire spread in buildings. Through the observations acquired in fires, it was concluded that reducing spread rates enabled safe egress and a more effective intervention by the fire service. Thus, different forms of compartmentalization permeated through most prescriptive codes. Once fire behaviour within a compartment was conceptualized on the basis of scientific principles, the compartment fire framework became a means to establish, under certain specific circumstances, temperatures and thermal loads imposed by a fire to a building. This resulted not only in improved codes but also in a scientifically based methodology for establishing the thermal input from which to assess structural performance. The last decades have however seen an evolution of the built environment away from compartmentalization while the classic compartment fire framework has remained. Within this framework, while Regime I corresponds to the idealised experimental setups adopted by many of the researchers, the usually ignored Regime II is characteristic of open spaces and volumes, typical of contemporary architecture. This research project commences, through a review of classic literature by those regarded as the fathers of fire safety engineering, by revisiting the knowledge underpinning this seminal approach, and initiating the discussion of its continued relevance and applicability to an increasingly non-compartmentalised built environment. Compartment fires are extremely complex processes. Nevertheless, when treating the theoretical problem with sufficient accuracy, simple mathematical approaches can be extremely informative and serve as the background to more complex methodologies. In this context, the project introduces the problem of the compartment fire in its full complexity before discussing some simplifications typically assumed when representing the actual problem for design purposes. Further, despite the detailed experimental and theoretical background behind analytical formulations in the classic compartment fire framework, their development is revisited to establish the extent to which they can be applied. In this way, the range of validity of the classic framework is characterized, clarifying the limitations of existing design methods based on this framework, and identifying the areas where further research and extension is necessary. Given the importance of counting on simple analytical formulations at the early design stage when dealing with atypical architectural designs in today’s fire safety practice, an elementary theoretical compartment fire framework is elaborated with the aim of enveloping traditional as well as contemporary architectural layouts. This gave way to the development of a new set of regime of behaviour definitions, in addition to – and falling in-between – the classic Regime I and Regime II fullydeveloped compartment fire behaviours. With the aim of filling this gap of knowledge empirically and characterizing these additional behaviours, a series of small and large-scale tests are presented. The results demonstrate complex behaviours that cannot be described in terms of the classic framework. This evidences the great need to conduct research that provides physical insight into the dynamics of a fire in spaces that deviate from the small quasi-cubic enclosure – the natural consequence of compartmentalization – that was typically adopted throughout the original work that resulted in the data which validated the classic compartment fire framework. Overall, this project aims to inform and encourage the discussion of the existence of a broader compartment fire framework, where the historical Regimes I and II are limiting cases of a vaster fire behaviour which is intimately linked to the geometry of the compartment, the ventilation conditions, and the available fuel. While the classic compartment fire framework is still a robust tool, it is only one piece in the puzzle of approaching and resolving the fire problem in a building in a holistic way. The relevance of this discussion is apparent in face of contemporary architecture and infrastructure.