Abstract How can one say that current systems of Air Traffic Management (ATM) systems are safe and resilient? What is the current knowledge and understanding of these systems? Why is it necessary to know them better? The Model of ATM Reality In Action (MARIA) is a knowledge database and an automation framework developed by NAV Portugal to answer the above questions and to provide a sound base for safety analysis, namely by describing the whole system and the interdependencies between its functions. It is coded as a graph giving additional possibilities of automatic analysis. It has undergone extensive validation and is considered mature. Experimental work has been done on hazard and cause identification with success. MARIA is neither a business nor safety model. It is built to allow several perspectives of analysis depending on the aim at hand. It may be used in simulations to identify strong and weak points, to evaluate risk mitigation strategies and for other purposes. Modeling the dynamics and the unknown are areas of planned study using the emergent methods for the modeling of complex systems. Having a repository covering all that is done to provide an ATM service, in a systematic and uniform way, structured in a topdown manner, with the human as an integral part of the system, allowing automation of analysis and representation are significant advantages to improve understanding, safety and resilience of the ATM services. Sharing this knowledge database and adding the contributions from other models and interested partners will be beneficial for the ATM world. Keywords- System modeling; safety; resilience; complex systems; ATM functions; ATM information flows